| Message ID | 20241030134912.515725-1-roypat@amazon.co.uk (mailing list archive) |
|---|---|
| Headers | show |
| Series | Direct Map Removal for guest_memfd | expand |
On 30.10.24 14:49, Patrick Roy wrote: > Unmapping virtual machine guest memory from the host kernel's direct map > is a successful mitigation against Spectre-style transient execution > issues: If the kernel page tables do not contain entries pointing to > guest memory, then any attempted speculative read through the direct map > will necessarily be blocked by the MMU before any observable > microarchitectural side-effects happen. This means that Spectre-gadgets > and similar cannot be used to target virtual machine memory. Roughly 60% > of speculative execution issues fall into this category [1, Table 1]. > > This patch series extends guest_memfd with the ability to remove its > memory from the host kernel's direct map, to be able to attain the above > protection for KVM guests running inside guest_memfd. > > === Changes to v2 === > > - Handle direct map removal for physically contiguous pages in arch code > (Mike R.) > - Track the direct map state in guest_memfd itself instead of at the > folio level, to prepare for huge pages support (Sean C.) > - Allow configuring direct map state of not-yet faulted in memory > (Vishal A.) > - Pay attention to alignment in ftrace structs (Steven R.) > > Most significantly, I've reduced the patch series to focus only on > direct map removal for guest_memfd for now, leaving the whole "how to do > non-CoCo VMs in guest_memfd" for later. If this separation is > acceptable, then I think I can drop the RFC tag in the next revision > (I've mainly kept it here because I'm not entirely sure what to do with > patches 3 and 4). Hi, keeping upcoming "shared and private memory in guest_memfd" in mind, I assume the focus would be to only remove the direct map for private memory? So in the current upstream state, you would only be removing the direct map for private memory, currently translating to "encrypted"/"protected" memory that is inaccessible either way already. Correct?
On Thu, 2024-10-31 at 09:50 +0000, David Hildenbrand wrote: > On 30.10.24 14:49, Patrick Roy wrote: >> Unmapping virtual machine guest memory from the host kernel's direct map >> is a successful mitigation against Spectre-style transient execution >> issues: If the kernel page tables do not contain entries pointing to >> guest memory, then any attempted speculative read through the direct map >> will necessarily be blocked by the MMU before any observable >> microarchitectural side-effects happen. This means that Spectre-gadgets >> and similar cannot be used to target virtual machine memory. Roughly 60% >> of speculative execution issues fall into this category [1, Table 1]. >> >> This patch series extends guest_memfd with the ability to remove its >> memory from the host kernel's direct map, to be able to attain the above >> protection for KVM guests running inside guest_memfd. >> >> === Changes to v2 === >> >> - Handle direct map removal for physically contiguous pages in arch code >> (Mike R.) >> - Track the direct map state in guest_memfd itself instead of at the >> folio level, to prepare for huge pages support (Sean C.) >> - Allow configuring direct map state of not-yet faulted in memory >> (Vishal A.) >> - Pay attention to alignment in ftrace structs (Steven R.) >> >> Most significantly, I've reduced the patch series to focus only on >> direct map removal for guest_memfd for now, leaving the whole "how to do >> non-CoCo VMs in guest_memfd" for later. If this separation is >> acceptable, then I think I can drop the RFC tag in the next revision >> (I've mainly kept it here because I'm not entirely sure what to do with >> patches 3 and 4). > > Hi, > > keeping upcoming "shared and private memory in guest_memfd" in mind, I > assume the focus would be to only remove the direct map for private memory? > > So in the current upstream state, you would only be removing the direct > map for private memory, currently translating to "encrypted"/"protected" > memory that is inaccessible either way already. > > Correct? Yea, with the upcomming "shared and private" stuff, I would expect the the shared<->private conversions would call the routines from patch 3 to restore direct map entries on private->shared, and zap them on shared->private. But as you said, the current upstream state has no notion of "shared" memory in guest_memfd, so everything is private and thus everything is direct map removed (although it is indeed already inaccessible anyway for TDX and friends. That's what makes this patch series a bit awkward :( ) > -- > Cheers, > > David / dhildenb > Best, Patrick
On 2024-10-31 at 10:42+0000 Patrick Roy wrote: > On Thu, 2024-10-31 at 09:50 +0000, David Hildenbrand wrote: > > On 30.10.24 14:49, Patrick Roy wrote: > >> Most significantly, I've reduced the patch series to focus only on > >> direct map removal for guest_memfd for now, leaving the whole "how to do > >> non-CoCo VMs in guest_memfd" for later. If this separation is > >> acceptable, then I think I can drop the RFC tag in the next revision > >> (I've mainly kept it here because I'm not entirely sure what to do with > >> patches 3 and 4). > > > > Hi, > > > > keeping upcoming "shared and private memory in guest_memfd" in mind, I > > assume the focus would be to only remove the direct map for private memory? > > > > So in the current upstream state, you would only be removing the direct > > map for private memory, currently translating to "encrypted"/"protected" > > memory that is inaccessible either way already. > > > > Correct? > > Yea, with the upcomming "shared and private" stuff, I would expect the > the shared<->private conversions would call the routines from patch 3 to > restore direct map entries on private->shared, and zap them on > shared->private. > > But as you said, the current upstream state has no notion of "shared" > memory in guest_memfd, so everything is private and thus everything is > direct map removed (although it is indeed already inaccessible anyway > for TDX and friends. That's what makes this patch series a bit awkward > :( ) TDX and SEV encryption happens between the core and main memory, so cached guest data we're most concerned about for transient execution attacks isn't necessarily inaccessible. I'd be interested what Intel, AMD, and other folks think on this, but I think direct map removal is worthwhile for CoCo cases as well. Derek
+David Kaplan On Thu, Oct 31, 2024, Derek Manwaring wrote: > On 2024-10-31 at 10:42+0000 Patrick Roy wrote: > > On Thu, 2024-10-31 at 09:50 +0000, David Hildenbrand wrote: > > > On 30.10.24 14:49, Patrick Roy wrote: > > >> Most significantly, I've reduced the patch series to focus only on > > >> direct map removal for guest_memfd for now, leaving the whole "how to do > > >> non-CoCo VMs in guest_memfd" for later. If this separation is > > >> acceptable, then I think I can drop the RFC tag in the next revision > > >> (I've mainly kept it here because I'm not entirely sure what to do with > > >> patches 3 and 4). > > > > > > Hi, > > > > > > keeping upcoming "shared and private memory in guest_memfd" in mind, I > > > assume the focus would be to only remove the direct map for private memory? > > > > > > So in the current upstream state, you would only be removing the direct > > > map for private memory, currently translating to "encrypted"/"protected" > > > memory that is inaccessible either way already. > > > > > > Correct? > > > > Yea, with the upcomming "shared and private" stuff, I would expect the > > the shared<->private conversions would call the routines from patch 3 to > > restore direct map entries on private->shared, and zap them on > > shared->private. > > > > But as you said, the current upstream state has no notion of "shared" > > memory in guest_memfd, so everything is private and thus everything is > > direct map removed (although it is indeed already inaccessible anyway > > for TDX and friends. That's what makes this patch series a bit awkward > > :( ) > > TDX and SEV encryption happens between the core and main memory, so > cached guest data we're most concerned about for transient execution > attacks isn't necessarily inaccessible. > > I'd be interested what Intel, AMD, and other folks think on this, but I > think direct map removal is worthwhile for CoCo cases as well. Removal of the direct map entries for guest private PFNs likely won't affect the ability of an attacker to glean information from the unencrypted data that's in the CPU caches, at least not on x86. Both TDX and SEV steal physical address bit(s) for tagging encrypted memory, and unless things have changed on recent AMD microarchitectures (I'm 99.9% certain Intel CPUs haven't changed), those stolen address bits are propagated into the caches. I.e. the encrypted and unencrypted forms of a given PFN are actually two different physical addresses under the hood. I don't actually know how SEV uses the stolen PA bits though. I don't see how it simply be the ASID, because IIUC, AMD CPUs allow for more unique SEV-capable ASIDs than uniquely addressable PAs by the number of stolen bits. But I would be very surprised if the tag for the cache isn't guaranteed to be unique per encryption key. David?
On 10/31/24 17:10, Manwaring, Derek wrote: > TDX and SEV encryption happens between the core and main memory, so > cached guest data we're most concerned about for transient execution > attacks isn't necessarily inaccessible. > > I'd be interested what Intel, AMD, and other folks think on this, but I > think direct map removal is worthwhile for CoCo cases as well. I'm not sure specifically which attacks you have in mind. Also, don't forget that TDX doesn't get any intra-system security from encryption itself. All that the encryption does is prevent some physical attacks. The main thing I think you want to keep in mind is mentioned in the "TDX Module v1.5 Base Architecture Specification"[1]: > Any software except guest TD or TDX module must not be able to > speculatively or non-speculatively access TD private memory, That's a pretty broad claim and it involves mitigations in hardware and the TDX module. I _think_ you might be thinking of attacks like MDS where some random microarchitectural buffer contains guest data after a VM exit and then an attacker extracts it. Direct map removal doesn't affect these buffers and doesn't mitigate an attacker getting the data out. TDX relies on other defenses. As for the CPU caches, direct map removal doesn't help or hurt anything. A malicious VMM would just map the guest data if it could and try to extract it if that were possible. If the attack is mitigated when the data is _mapped_, then it's certainly not possible _unmapped_. So why bother with direct map removal for TDX? A VMM write to TD private data causes machine checks. So any kernel bug that even accidentally writes to kernel memory can bring the whole system down. Not nice. 1. https://cdrdv2.intel.com/v1/dl/getContent/733575
+Elena On 2024-11-01 at 16:06+0000, Dave Hansen wrote: > On 10/31/24 17:10, Manwaring, Derek wrote: > > TDX and SEV encryption happens between the core and main memory, so > > cached guest data we're most concerned about for transient execution > > attacks isn't necessarily inaccessible. > > > > I'd be interested what Intel, AMD, and other folks think on this, but I > > think direct map removal is worthwhile for CoCo cases as well. > > I'm not sure specifically which attacks you have in mind. [...] > > I _think_ you might be thinking of attacks like MDS where some random > microarchitectural buffer contains guest data after a VM exit and then > an attacker extracts it. Direct map removal doesn't affect these > buffers and doesn't mitigate an attacker getting the data out. Right, the only attacks we can thwart with direct map removal are transient execution attacks on the host kernel whose leak origin is "Mapped memory" in Table 1 of the Quarantine paper [2]. Maybe the simplest hypothetical to consider here is a new spectre v1 gadget in the host kernel. > The main thing I think you want to keep in mind is mentioned in the "TDX > Module v1.5 Base Architecture Specification"[1]: > > > Any software except guest TD or TDX module must not be able to > > speculatively or non-speculatively access TD private memory, > > That's a pretty broad claim and it involves mitigations in hardware and > the TDX module. > > 1. https://cdrdv2.intel.com/v1/dl/getContent/733575 Thank you, I hadn't seen that. That is a very strong claim as far as preventing speculative access; I didn't realize Intel claimed that about TDX. The comma followed by "to detect if a prior corruption attempt was successful" makes me wonder a bit if the statement is not quite as broad as it sounds, but maybe that's just meant to relate it to the integrity section? > If the attack is mitigated when the > data is _mapped_, then it's > certainly not possible _unmapped_. > > So why bother with direct map removal for TDX? A VMM write to TD > private data causes machine checks. So any kernel bug that even > accidentally writes to kernel memory can bring the whole system down. > Not nice. Fair enough. It hasn't been clear to me if there is a machine check when the host kernel accesses guest memory only transiently. I was assuming there is not. But if other mitigations completely prevent even speculative access of TD private memory like you're saying, then agree nothing to gain from direct map removal in the TDX case. Derek [2] https://download.vusec.net/papers/quarantine_raid23.pdf
On 11/1/24 09:56, Manwaring, Derek wrote: ... >>> Any software except guest TD or TDX module must not be able to >>> speculatively or non-speculatively access TD private memory, >> >> That's a pretty broad claim and it involves mitigations in hardware and >> the TDX module. >> >> 1. https://cdrdv2.intel.com/v1/dl/getContent/733575 > > Thank you, I hadn't seen that. That is a very strong claim as far as > preventing speculative access; I didn't realize Intel claimed that about > TDX. The comma followed by "to detect if a prior corruption attempt was > successful" makes me wonder a bit if the statement is not quite as broad > as it sounds, but maybe that's just meant to relate it to the integrity > section? I think it's just relating it to the integrity section. >> If the attack is mitigated when the > data is _mapped_, then it's >> certainly not possible _unmapped_. >> >> So why bother with direct map removal for TDX? A VMM write to TD >> private data causes machine checks. So any kernel bug that even >> accidentally writes to kernel memory can bring the whole system down. >> Not nice. > > Fair enough. It hasn't been clear to me if there is a machine check when > the host kernel accesses guest memory only transiently. I was assuming > there is not. Previous generations of hardware have had some nastiness in this area. Speculative accesses were (I think) logged in the machine check banks, but wouldn't raise an #MC. I believe TDX-capable hardware won't even log speculative accesses. > But if other mitigations completely prevent even speculative access > of TD private memory like you're saying, then agree nothing to gain > from direct map removal in the TDX case. Remember, guest unmapping is done in the VMM. The VMM is not trusted in the TDX (or SEV-SNP) model. If any VMM can harm the protections on guest memory, then we have a big problem. That isn't to say big problem can't happen. Say some crazy attack comes to light where the VMM can attack TDX if the VMM has mapping for a guest (or TDX module) memory. Crazier things have happened, and guest unmapping _would_ help there, if you trusted the VMM. Basically, I think guest unmapping only helps system security as a whole if you must _already_ trust the VMM.
On 2024-11-01 at 17:20+0000, Dave Hansen wrote: > On 11/1/24 09:56, Manwaring, Derek wrote: > > But if other mitigations completely prevent even speculative access > > of TD private memory like you're saying, then agree nothing to gain > > from direct map removal in the TDX case. > Remember, guest unmapping is done in the VMM. The VMM is not trusted in > the TDX (or SEV-SNP) model. If any VMM can harm the protections on > guest memory, then we have a big problem. > > That isn't to say big problem can't happen. Say some crazy attack comes > to light where the VMM can attack TDX if the VMM has mapping for a guest > (or TDX module) memory. Crazier things have happened, and guest > unmapping _would_ help there, if you trusted the VMM. > > Basically, I think guest unmapping only helps system security as a whole > if you must _already_ trust the VMM. Yeah that makes a lot of sense. I just view the ideal outcome as a composition of strong, independent defenses. So as a guest you have the confidentiality and integrity guarantees of the hardware, *and* you have an up-to-date, good-hygiene (albeit not attested) host kernel just in case some crazy attack/gap comes up. From that standpoint I'm still tempted to turn the question around a bit for the host kernel's perspective. Like if the host kernel should not (and indeed cannot with TDX controls in place) access guest private memory, why not remove it from the direct map? Derek
[AMD Official Use Only - AMD Internal Distribution Only] > -----Original Message----- > From: Sean Christopherson <seanjc@google.com> > Sent: Friday, November 1, 2024 10:18 AM > To: Derek Manwaring <derekmn@amazon.com> > Cc: roypat@amazon.co.uk; ackerleytng@google.com; > agordeev@linux.ibm.com; aou@eecs.berkeley.edu; > borntraeger@linux.ibm.com; bp@alien8.de; catalin.marinas@arm.com; > chenhuacai@kernel.org; corbet@lwn.net; dave.hansen@linux.intel.com; > david@redhat.com; gerald.schaefer@linux.ibm.com; gor@linux.ibm.com; > graf@amazon.com; hca@linux.ibm.com; hpa@zytor.com; > jgowans@amazon.com; jthoughton@google.com; kalyazin@amazon.com; > kernel@xen0n.name; kvm@vger.kernel.org; linux-arm- > kernel@lists.infradead.org; linux-doc@vger.kernel.org; linux- > kernel@vger.kernel.org; linux-kselftest@vger.kernel.org; linux- > mm@kvack.org; linux-riscv@lists.infradead.org; linux-s390@vger.kernel.org; > linux-trace-kernel@vger.kernel.org; loongarch@lists.linux.dev; > luto@kernel.org; mathieu.desnoyers@efficios.com; mhiramat@kernel.org; > mingo@redhat.com; palmer@dabbelt.com; paul.walmsley@sifive.com; > pbonzini@redhat.com; peterz@infradead.org; quic_eberman@quicinc.com; > rostedt@goodmis.org; rppt@kernel.org; shuah@kernel.org; > svens@linux.ibm.com; tabba@google.com; tglx@linutronix.de; > vannapurve@google.com; will@kernel.org; x86@kernel.org; > xmarcalx@amazon.com; Kaplan, David <David.Kaplan@amd.com> > Subject: Re: [RFC PATCH v3 0/6] Direct Map Removal for guest_memfd > > Caution: This message originated from an External Source. Use proper > caution when opening attachments, clicking links, or responding. > > > +David Kaplan > > On Thu, Oct 31, 2024, Derek Manwaring wrote: > > On 2024-10-31 at 10:42+0000 Patrick Roy wrote: > > > On Thu, 2024-10-31 at 09:50 +0000, David Hildenbrand wrote: > > > > On 30.10.24 14:49, Patrick Roy wrote: > > > >> Most significantly, I've reduced the patch series to focus only > > > >> on direct map removal for guest_memfd for now, leaving the whole > > > >> "how to do non-CoCo VMs in guest_memfd" for later. If this > > > >> separation is acceptable, then I think I can drop the RFC tag in > > > >> the next revision (I've mainly kept it here because I'm not > > > >> entirely sure what to do with patches 3 and 4). > > > > > > > > Hi, > > > > > > > > keeping upcoming "shared and private memory in guest_memfd" in > > > > mind, I assume the focus would be to only remove the direct map for > private memory? > > > > > > > > So in the current upstream state, you would only be removing the > > > > direct map for private memory, currently translating to > "encrypted"/"protected" > > > > memory that is inaccessible either way already. > > > > > > > > Correct? > > > > > > Yea, with the upcomming "shared and private" stuff, I would expect > > > the the shared<->private conversions would call the routines from > > > patch 3 to restore direct map entries on private->shared, and zap > > > them on > > > shared->private. > > > > > > But as you said, the current upstream state has no notion of "shared" > > > memory in guest_memfd, so everything is private and thus everything > > > is direct map removed (although it is indeed already inaccessible > > > anyway for TDX and friends. That's what makes this patch series a > > > bit awkward :( ) > > > > TDX and SEV encryption happens between the core and main memory, so > > cached guest data we're most concerned about for transient execution > > attacks isn't necessarily inaccessible. > > > > I'd be interested what Intel, AMD, and other folks think on this, but > > I think direct map removal is worthwhile for CoCo cases as well. > > Removal of the direct map entries for guest private PFNs likely won't affect > the ability of an attacker to glean information from the unencrypted data > that's in the CPU caches, at least not on x86. Both TDX and SEV steal physical > address > bit(s) for tagging encrypted memory, and unless things have changed on > recent AMD microarchitectures (I'm 99.9% certain Intel CPUs haven't > changed), those stolen address bits are propagated into the caches. I.e. the > encrypted and unencrypted forms of a given PFN are actually two different > physical addresses under the hood. > > I don't actually know how SEV uses the stolen PA bits though. I don't see > how it simply be the ASID, because IIUC, AMD CPUs allow for more unique > SEV-capable ASIDs than uniquely addressable PAs by the number of stolen > bits. But I would be very surprised if the tag for the cache isn't guaranteed to > be unique per encryption key. > > David? How the stolen PA bits are used is a microarchitectural implementation detail. It is true that the tag will be unique per encryption key. Beyond that, I'm not sure what other details are relevant to SW. --David Kaplan
On 11/1/24 11:31, Manwaring, Derek wrote: >>From that standpoint I'm still tempted to turn the question around a bit > for the host kernel's perspective. Like if the host kernel should not > (and indeed cannot with TDX controls in place) access guest private > memory, why not remove it from the direct map? Pretend that the machine check warts aren't there. It costs performance and complexity, for an only theoretical gain. This is especially true for a VMM that's not doing a just doing confidential guests. You fracture the direct map to pieces forever (for now).
On 2024-11-01 at 18:43+0000, Dave Hansen wrote: > On 11/1/24 11:31, Manwaring, Derek wrote: > > From that standpoint I'm still tempted to turn the question around a bit > > for the host kernel's perspective. Like if the host kernel should not > > (and indeed cannot with TDX controls in place) access guest private > > memory, why not remove it from the direct map? > > Pretend that the machine check warts aren't there. > > It costs performance and complexity, for an only theoretical gain. This > is especially true for a VMM that's not doing a just doing confidential > guests. You fracture the direct map to pieces forever (for now). I'm hopeful we'll navigate the complexity in a worthwhile way for the non-CoCo case. Assuming we get there and have the option to remove from direct map, users with CoCo hardware could choose if they want to do both on their host. For me that's a sensible choice, but maybe that's just me. As far as performance, are you talking about just the fracturing or something beyond that? The data Mike brought to LSFMMBPF 2023 showed the perf impact from direct map fragmentation for memfd_secret isn't "that bad" [1]. Derek [1] https://lwn.net/Articles/931406/
On 11/1/24 12:29, Manwaring, Derek wrote: > As far as performance, are you talking about just the fracturing or > something beyond that? The data Mike brought to LSFMMBPF 2023 showed the > perf impact from direct map fragmentation for memfd_secret isn't "that > bad" [1]. Just the fracturing. Mike's data are interesting, but I still think we're pretty far away from saying that the kernel doesn't need large mappings.
> > +Elena > > On 2024-11-01 at 16:06+0000, Dave Hansen wrote: > > On 10/31/24 17:10, Manwaring, Derek wrote: > > > TDX and SEV encryption happens between the core and main memory, so > > > cached guest data we're most concerned about for transient execution > > > attacks isn't necessarily inaccessible. > > > > > > I'd be interested what Intel, AMD, and other folks think on this, but I > > > think direct map removal is worthwhile for CoCo cases as well. > > > > I'm not sure specifically which attacks you have in mind. [...] > > > > I _think_ you might be thinking of attacks like MDS where some random > > microarchitectural buffer contains guest data after a VM exit and then > > an attacker extracts it. Direct map removal doesn't affect these > > buffers and doesn't mitigate an attacker getting the data out. > > Right, the only attacks we can thwart with direct map removal are > transient execution attacks on the host kernel whose leak origin is > "Mapped memory" in Table 1 of the Quarantine paper [2]. Maybe the > simplest hypothetical to consider here is a new spectre v1 gadget in the > host kernel. > > > The main thing I think you want to keep in mind is mentioned in the "TDX > > Module v1.5 Base Architecture Specification"[1]: > > > > > Any software except guest TD or TDX module must not be able to > > > speculatively or non-speculatively access TD private memory, > > > > That's a pretty broad claim and it involves mitigations in hardware and > > the TDX module. > > > > 1. https://cdrdv2.intel.com/v1/dl/getContent/733575 > > Thank you, I hadn't seen that. That is a very strong claim as far as > preventing speculative access; I didn't realize Intel claimed that about > TDX. The comma followed by "to detect if a prior corruption attempt was > successful" makes me wonder a bit if the statement is not quite as broad > as it sounds, but maybe that's just meant to relate it to the integrity > section? This statement *is* for integrity section. We have a separate TDX guidance on side-channels (including speculative) [3] and some speculative attacks that affect confidentiality (for example spectre v1) are listed as not covered by TDX but remaining SW responsibility (as they are now). [3] https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/best-practices/trusted-domain-security-guidance-for-developers.html Best Regards, Elena.
On 31.10.24 11:42, Patrick Roy wrote: > On Thu, 2024-10-31 at 09:50 +0000, David Hildenbrand wrote: >> On 30.10.24 14:49, Patrick Roy wrote: >>> Unmapping virtual machine guest memory from the host kernel's direct map >>> is a successful mitigation against Spectre-style transient execution >>> issues: If the kernel page tables do not contain entries pointing to >>> guest memory, then any attempted speculative read through the direct map >>> will necessarily be blocked by the MMU before any observable >>> microarchitectural side-effects happen. This means that Spectre-gadgets >>> and similar cannot be used to target virtual machine memory. Roughly 60% >>> of speculative execution issues fall into this category [1, Table 1]. >>> >>> This patch series extends guest_memfd with the ability to remove its >>> memory from the host kernel's direct map, to be able to attain the above >>> protection for KVM guests running inside guest_memfd. >>> >>> === Changes to v2 === >>> >>> - Handle direct map removal for physically contiguous pages in arch code >>> (Mike R.) >>> - Track the direct map state in guest_memfd itself instead of at the >>> folio level, to prepare for huge pages support (Sean C.) >>> - Allow configuring direct map state of not-yet faulted in memory >>> (Vishal A.) >>> - Pay attention to alignment in ftrace structs (Steven R.) >>> >>> Most significantly, I've reduced the patch series to focus only on >>> direct map removal for guest_memfd for now, leaving the whole "how to do >>> non-CoCo VMs in guest_memfd" for later. If this separation is >>> acceptable, then I think I can drop the RFC tag in the next revision >>> (I've mainly kept it here because I'm not entirely sure what to do with >>> patches 3 and 4). >> >> Hi, >> >> keeping upcoming "shared and private memory in guest_memfd" in mind, I >> assume the focus would be to only remove the direct map for private memory? >> >> So in the current upstream state, you would only be removing the direct >> map for private memory, currently translating to "encrypted"/"protected" >> memory that is inaccessible either way already. >> >> Correct? > > Yea, with the upcomming "shared and private" stuff, I would expect the > the shared<->private conversions would call the routines from patch 3 to > restore direct map entries on private->shared, and zap them on > shared->private. I wanted to follow-up to the discussion we had in the bi-weekly call. We talked about shared (faultable) vs. private (unfaultable), and how it would interact with the directmap patches here. As discussed, having private (unfaultable) memory with the direct-map removed and shared (faultable) memory with the direct-mapping can make sense for non-TDX/AMD-SEV/... non-CoCo use cases. Not sure about CoCo, the discussion here seems to indicate that it might currently not be required. So one thing we could do is that shared (faultable) will have a direct mapping and be gup-able and private (unfaultable) memory will not have a direct mapping and is, by design, not gup-able. Maybe it could make sense to not have a direct map for all guest_memfd memory, making it behave like secretmem (and it would be easy to implement)? But I'm not sure if that is really desirable in VM context. Having a mixture of "has directmap" and "has no directmap" for shared (faultable) memory should not be done. Similarly, private memory really should stay "unfaultable". I think one of the points raised during the bi-weekly call was that using a viommu/swiotlb might be the right call, such that all memory can be considered private (unfaultable) that is not explicitly shared/expected to be modified by the hypervisor (-> faultable, -> GUP-able). Further, I think Sean had some good points why we should explore that direction, but I recall that there were some issue to be sorted out (interpreted instructions requiring direct map when accessing "private" memory?), not sure if that is already working/can be made working in KVM. What's your opinion after the call and the next step for use cases like you have in mind (IIRC firecracker, which wants to not have the direct-map for guest memory where it can be avoided)?
Hi David, On 11/4/24 12:18, David Hildenbrand wrote: > On 31.10.24 11:42, Patrick Roy wrote: >> On Thu, 2024-10-31 at 09:50 +0000, David Hildenbrand wrote: >>> On 30.10.24 14:49, Patrick Roy wrote: >>>> Unmapping virtual machine guest memory from the host kernel's direct map >>>> is a successful mitigation against Spectre-style transient execution >>>> issues: If the kernel page tables do not contain entries pointing to >>>> guest memory, then any attempted speculative read through the direct map >>>> will necessarily be blocked by the MMU before any observable >>>> microarchitectural side-effects happen. This means that Spectre-gadgets >>>> and similar cannot be used to target virtual machine memory. Roughly 60% >>>> of speculative execution issues fall into this category [1, Table 1]. >>>> >>>> This patch series extends guest_memfd with the ability to remove its >>>> memory from the host kernel's direct map, to be able to attain the above >>>> protection for KVM guests running inside guest_memfd. >>>> >>>> === Changes to v2 === >>>> >>>> - Handle direct map removal for physically contiguous pages in arch code >>>> (Mike R.) >>>> - Track the direct map state in guest_memfd itself instead of at the >>>> folio level, to prepare for huge pages support (Sean C.) >>>> - Allow configuring direct map state of not-yet faulted in memory >>>> (Vishal A.) >>>> - Pay attention to alignment in ftrace structs (Steven R.) >>>> >>>> Most significantly, I've reduced the patch series to focus only on >>>> direct map removal for guest_memfd for now, leaving the whole "how to do >>>> non-CoCo VMs in guest_memfd" for later. If this separation is >>>> acceptable, then I think I can drop the RFC tag in the next revision >>>> (I've mainly kept it here because I'm not entirely sure what to do with >>>> patches 3 and 4). >>> >>> Hi, >>> >>> keeping upcoming "shared and private memory in guest_memfd" in mind, I >>> assume the focus would be to only remove the direct map for private memory? >>> >>> So in the current upstream state, you would only be removing the direct >>> map for private memory, currently translating to "encrypted"/"protected" >>> memory that is inaccessible either way already. >>> >>> Correct? >> >> Yea, with the upcomming "shared and private" stuff, I would expect the >> the shared<->private conversions would call the routines from patch 3 to >> restore direct map entries on private->shared, and zap them on >> shared->private. > > I wanted to follow-up to the discussion we had in the bi-weekly call. Thanks for summarizing! > We talked about shared (faultable) vs. private (unfaultable), and how it > would interact with the directmap patches here. > > As discussed, having private (unfaultable) memory with the direct-map > removed and shared (faultable) memory with the direct-mapping can make > sense for non-TDX/AMD-SEV/... non-CoCo use cases. Not sure about CoCo, > the discussion here seems to indicate that it might currently not be > required. > > So one thing we could do is that shared (faultable) will have a direct > mapping and be gup-able and private (unfaultable) memory will not have a > direct mapping and is, by design, not gup-able.> > Maybe it could make sense to not have a direct map for all guest_memfd > memory, making it behave like secretmem (and it would be easy to > implement)? But I'm not sure if that is really desirable in VM context. This would work for us (in this scenario, the swiotlb areas would be "traditional" memory, e.g. set to shared via mem attributes instead of "shared" inside KVM), it's kinda what I had prototyped in my v1 of this series (well, we'd need to figure out how to get the mappings of gmem back into KVM, since in this setup, short-circuiting it into userspace_addr wouldn't work, unless we banish swiotlb into a different memslot altogether somehow). But I don't think it'd work for pKVM, iirc they need GUP on gmem, and also want direct map removal (... but maybe, the gmem VMA for non-CoCo usecase and the gmem VMA for pKVM could be behave differently? non-CoCo gets essentially memfd_secret, pKVM gets GUP+no faults of private mem). > Having a mixture of "has directmap" and "has no directmap" for shared > (faultable) memory should not be done. Similarly, private memory really > should stay "unfaultable". You've convinced me that having both GUP-able and non GUP-able memory in the same VMA will be tricky. However, I'm less convinced on why private memory should stay unfaultable; only that it shouldn't be faultable into a VMA that also allows GUP. Can we have two VMAs? One that disallows GUP, but allows userspace access to shared and private, and one that allows GUP, but disallows accessing private memory? Maybe via some `PROT_NOGUP` flag to `mmap`? I guess this is a slightly different spin of the above idea. > I think one of the points raised during the bi-weekly call was that > using a viommu/swiotlb might be the right call, such that all memory can > be considered private (unfaultable) that is not explicitly > shared/expected to be modified by the hypervisor (-> faultable, -> > GUP-able). > > Further, I think Sean had some good points why we should explore that > direction, but I recall that there were some issue to be sorted out > (interpreted instructions requiring direct map when accessing "private" > memory?), not sure if that is already working/can be made working in KVM. Yeah, the big one is MMIO instruction emulation on x86, which does guest page table walks and instruction fetch (and particularly the latter cannot be known ahead-of-time by the guest, aka cannot be explicitly "shared"). That's what the majority of my v2 series was about. For traditional memslots, KVM handles these via get_user and friends, but if we don't have a VMA that allows faulting all of gmem, then that's impossible, and we're in "temporarily restore direct map" land. Which comes with significantly performance penalties due to TLB flushes. > What's your opinion after the call and the next step for use cases like > you have in mind (IIRC firecracker, which wants to not have the > direct-map for guest memory where it can be avoided)? Yea, the usecase is for Firecracker to not have direct map entries for guest memory, unless needed for I/O (-> swiotlb). As for next steps, let's determine once and for all if we can do the KVM-internal guest memory accesses for MMIO emulation through userspace mappings (although if we can't I'll have some serious soul-searching to do, because all other solutions we talked about so far also have fairly big drawbacks; on-demand direct map reinsertion has terrible performance, protection keys would limit us to 15 VMs on the host, and the page table swapping runs into problems with NMIs if I understood Sean correctly last Thursday :( ). > -- > Cheers, > > David / dhildenb Best, Patrick
>> We talked about shared (faultable) vs. private (unfaultable), and how it >> would interact with the directmap patches here. >> >> As discussed, having private (unfaultable) memory with the direct-map >> removed and shared (faultable) memory with the direct-mapping can make >> sense for non-TDX/AMD-SEV/... non-CoCo use cases. Not sure about CoCo, >> the discussion here seems to indicate that it might currently not be >> required. >> >> So one thing we could do is that shared (faultable) will have a direct >> mapping and be gup-able and private (unfaultable) memory will not have a >> direct mapping and is, by design, not gup-able.> >> Maybe it could make sense to not have a direct map for all guest_memfd >> memory, making it behave like secretmem (and it would be easy to >> implement)? But I'm not sure if that is really desirable in VM context. > > This would work for us (in this scenario, the swiotlb areas would be > "traditional" memory, e.g. set to shared via mem attributes instead of > "shared" inside KVM), it's kinda what I had prototyped in my v1 of this > series (well, we'd need to figure out how to get the mappings of gmem > back into KVM, since in this setup, short-circuiting it into > userspace_addr wouldn't work, unless we banish swiotlb into a different > memslot altogether somehow). Right. > But I don't think it'd work for pKVM, iirc > they need GUP on gmem, and also want direct map removal (... but maybe, > the gmem VMA for non-CoCo usecase and the gmem VMA for pKVM could be > behave differently? non-CoCo gets essentially memfd_secret, pKVM gets > GUP+no faults of private mem). Good question. So far my perception was that the directmap removal on "private/unfaultable" would be sufficient. > >> Having a mixture of "has directmap" and "has no directmap" for shared >> (faultable) memory should not be done. Similarly, private memory really >> should stay "unfaultable". > > You've convinced me that having both GUP-able and non GUP-able > memory in the same VMA will be tricky. However, I'm less convinced on > why private memory should stay unfaultable; only that it shouldn't be > faultable into a VMA that also allows GUP. Can we have two VMAs? One > that disallows GUP, but allows userspace access to shared and private, > and one that allows GUP, but disallows accessing private memory? Maybe > via some `PROT_NOGUP` flag to `mmap`? I guess this is a slightly > different spin of the above idea. What we are trying to achieve is making guest_memfd not behave completely different on that level for different "types" of VMs. So one of the goals should be to try to unify it as much as possible. shared -> faultable: GUP-able private -> unfaultable: unGUP-able And it makes sense, because a lot of future work will rely on some important properties: for example, if private memory cannot be faulted in + GUPed, core-MM will never have obtained valid references to such a page. There is no need to split large folios into smaller ones for tracking purposes; there is no need to maintain per-page refcounts and pincounts ... It doesn't mean that we cannot consider it if really required, but there really has to be a strong case for it, because it will all get really messy. For example, one issue is that a folio only has a single mapping (folio->mapping), and that is used in the GUP-fast path (no VMA) to determine whether GUP-fast is allowed or not. So you'd have to force everything through GUP-slow, where you could consider VMA properties :( It sounds quite suboptimal. I don't think multiple VMAs are what we really want. See below. > >> I think one of the points raised during the bi-weekly call was that >> using a viommu/swiotlb might be the right call, such that all memory can >> be considered private (unfaultable) that is not explicitly >> shared/expected to be modified by the hypervisor (-> faultable, -> >> GUP-able). >> >> Further, I think Sean had some good points why we should explore that >> direction, but I recall that there were some issue to be sorted out >> (interpreted instructions requiring direct map when accessing "private" >> memory?), not sure if that is already working/can be made working in KVM. > > Yeah, the big one is MMIO instruction emulation on x86, which does guest > page table walks and instruction fetch (and particularly the latter > cannot be known ahead-of-time by the guest, aka cannot be explicitly > "shared"). That's what the majority of my v2 series was about. For > traditional memslots, KVM handles these via get_user and friends, but if > we don't have a VMA that allows faulting all of gmem, then that's > impossible, and we're in "temporarily restore direct map" land. Which > comes with significantly performance penalties due to TLB flushes. Agreed. > >> What's your opinion after the call and the next step for use cases like >> you have in mind (IIRC firecracker, which wants to not have the >> direct-map for guest memory where it can be avoided)? > > Yea, the usecase is for Firecracker to not have direct map entries for > guest memory, unless needed for I/O (-> swiotlb). > > As for next steps, let's determine once and for all if we can do the > KVM-internal guest memory accesses for MMIO emulation through userspace > mappings (although if we can't I'll have some serious soul-searching to > do, because all other solutions we talked about so far also have fairly > big drawbacks; on-demand direct map reinsertion has terrible > performance So IIUC, KVM would have to access "unfaultable" guest_memfd memory using fd+offset, and that's problematic because "no-directmap". So you'd have to map+unmap the directmap repeatedly, and still expose it temporarily in the direct map to others. I see how that is undesirable, even when trying to cache hotspots (partly destroying the purpose of the directmap removal). Would a per-MM kernel mapping of these pages work, so KVM can access them? It sounds a bit like what is required for clean per-MM allocations [1]: establish a per-MM kernel mapping of (selected?) pages. Not necessarily all of them. Yes, we'd be avoiding VMAs, GUP, mapcounts, pincounts and everything involved with ordinary user mappings for these private/unfaultable thingies. Just like as discussed in, and similar to [1]. Just throwing it out there, maybe we really want to avoid the directmap (keep it unmapped) and maintain a per-mm mapping for a bunch of folios that can be easily removed when required by guest_memfd (ftruncate, conversion private->shared) on request. [1] https://lore.kernel.org/all/20240911143421.85612-1-faresx@amazon.de/T/#u
On 2024-11-04 at 08:33+0000, Elena Reshetova wrote: > This statement *is* for integrity section. We have a separate TDX guidance > on side-channels (including speculative) [3] and some speculative attacks > that affect confidentiality (for example spectre v1) are listed as not covered > by TDX but remaining SW responsibility (as they are now). Thanks for the additional info, Elena. Given that clarification, I definitely see direct map removal and TDX as complementary. Derek
> On 2024-11-04 at 08:33+0000, Elena Reshetova wrote: > > This statement *is* for integrity section. We have a separate TDX guidance > > on side-channels (including speculative) [3] and some speculative attacks > > that affect confidentiality (for example spectre v1) are listed as not covered > > by TDX but remaining SW responsibility (as they are now). > > Thanks for the additional info, Elena. Given that clarification, I > definitely see direct map removal and TDX as complementary. Jus to clarify to make sure my comment is not misunderstood. What I meant is that we cannot generally assume that confidentiality leaks from CoCo guests to host/VMM via speculative channels are completely impossible. Spectre V1 is a prime example of such a possible leak. Dave also elaborated on other potential vectors earlier. The usefulness of direct map removal for CoCo guests as a concrete mitigation for certain types of memory attacks must be precisely evaluated per each attack vector, attack vector direction (host -> guest, guest ->host, etc) and per each countermeasure that CoCo vendors provide for each such case. I don't know if there is any existing study that examines this for major CoCo vendors. I think this is what must be done for this work in order to have a strong reasoning for its usefulness. Best Regards, Elena.
Hi David, sorry for the late response, I ended up catching the flu last week and was out of commission for a while :( On Mon, 2024-11-04 at 21:30 +0000, David Hildenbrand wrote: >>> We talked about shared (faultable) vs. private (unfaultable), and how it >>> would interact with the directmap patches here. >>> >>> As discussed, having private (unfaultable) memory with the direct-map >>> removed and shared (faultable) memory with the direct-mapping can make >>> sense for non-TDX/AMD-SEV/... non-CoCo use cases. Not sure about CoCo, >>> the discussion here seems to indicate that it might currently not be >>> required. >>> >>> So one thing we could do is that shared (faultable) will have a direct >>> mapping and be gup-able and private (unfaultable) memory will not have a >>> direct mapping and is, by design, not gup-able.> >>> Maybe it could make sense to not have a direct map for all guest_memfd >>> memory, making it behave like secretmem (and it would be easy to >>> implement)? But I'm not sure if that is really desirable in VM context. >> >> This would work for us (in this scenario, the swiotlb areas would be >> "traditional" memory, e.g. set to shared via mem attributes instead of >> "shared" inside KVM), it's kinda what I had prototyped in my v1 of this >> series (well, we'd need to figure out how to get the mappings of gmem >> back into KVM, since in this setup, short-circuiting it into >> userspace_addr wouldn't work, unless we banish swiotlb into a different >> memslot altogether somehow). > > Right. "right" as in, "yes we could do that"? :p >> But I don't think it'd work for pKVM, iirc >> they need GUP on gmem, and also want direct map removal (... but maybe, >> the gmem VMA for non-CoCo usecase and the gmem VMA for pKVM could be >> behave differently? non-CoCo gets essentially memfd_secret, pKVM gets >> GUP+no faults of private mem). > > Good question. So far my perception was that the directmap removal on > "private/unfaultable" would be sufficient. > >> >>> Having a mixture of "has directmap" and "has no directmap" for shared >>> (faultable) memory should not be done. Similarly, private memory really >>> should stay "unfaultable". >> >> You've convinced me that having both GUP-able and non GUP-able >> memory in the same VMA will be tricky. However, I'm less convinced on >> why private memory should stay unfaultable; only that it shouldn't be >> faultable into a VMA that also allows GUP. Can we have two VMAs? One >> that disallows GUP, but allows userspace access to shared and private, >> and one that allows GUP, but disallows accessing private memory? Maybe >> via some `PROT_NOGUP` flag to `mmap`? I guess this is a slightly >> different spin of the above idea. > > What we are trying to achieve is making guest_memfd not behave > completely different on that level for different "types" of VMs. So one > of the goals should be to try to unify it as much as possible. > > shared -> faultable: GUP-able > private -> unfaultable: unGUP-able > > > And it makes sense, because a lot of future work will rely on some > important properties: for example, if private memory cannot be faulted > in + GUPed, core-MM will never have obtained valid references to such a > page. There is no need to split large folios into smaller ones for > tracking purposes; there is no need to maintain per-page refcounts and > pincounts ... > > It doesn't mean that we cannot consider it if really required, but there > really has to be a strong case for it, because it will all get really messy. > > For example, one issue is that a folio only has a single mapping > (folio->mapping), and that is used in the GUP-fast path (no VMA) to > determine whether GUP-fast is allowed or not. > > So you'd have to force everything through GUP-slow, where you could > consider VMA properties :( It sounds quite suboptimal. > > I don't think multiple VMAs are what we really want. See below. Ah, okay, I see. Thanks for explaining, this all makes a lot of sense to me now! >> >>> I think one of the points raised during the bi-weekly call was that >>> using a viommu/swiotlb might be the right call, such that all memory can >>> be considered private (unfaultable) that is not explicitly >>> shared/expected to be modified by the hypervisor (-> faultable, -> >>> GUP-able). >>> >>> Further, I think Sean had some good points why we should explore that >>> direction, but I recall that there were some issue to be sorted out >>> (interpreted instructions requiring direct map when accessing "private" >>> memory?), not sure if that is already working/can be made working in KVM. >> >> Yeah, the big one is MMIO instruction emulation on x86, which does guest >> page table walks and instruction fetch (and particularly the latter >> cannot be known ahead-of-time by the guest, aka cannot be explicitly >> "shared"). That's what the majority of my v2 series was about. For >> traditional memslots, KVM handles these via get_user and friends, but if >> we don't have a VMA that allows faulting all of gmem, then that's >> impossible, and we're in "temporarily restore direct map" land. Which >> comes with significantly performance penalties due to TLB flushes. > > Agreed. > >> >> What's your opinion after the call and the next step for use cases > like >>> you have in mind (IIRC firecracker, which wants to not have the >>> direct-map for guest memory where it can be avoided)? >> >> Yea, the usecase is for Firecracker to not have direct map entries for >> guest memory, unless needed for I/O (-> swiotlb). >> >> As for next steps, let's determine once and for all if we can do the >> KVM-internal guest memory accesses for MMIO emulation through userspace >> mappings (although if we can't I'll have some serious soul-searching to >> do, because all other solutions we talked about so far also have fairly >> big drawbacks; on-demand direct map reinsertion has terrible >> performance > So IIUC, KVM would have to access "unfaultable" guest_memfd memory using > fd+offset, and that's problematic because "no-directmap". > > So you'd have to map+unmap the directmap repeatedly, and still expose it > temporarily in the direct map to others. I see how that is undesirable, > even when trying to cache hotspots (partly destroying the purpose of the > directmap removal). > > > Would a per-MM kernel mapping of these pages work, so KVM can access them? > > It sounds a bit like what is required for clean per-MM allocations [1]: > establish a per-MM kernel mapping of (selected?) pages. Not necessarily > all of them. > > Yes, we'd be avoiding VMAs, GUP, mapcounts, pincounts and everything > involved with ordinary user mappings for these private/unfaultable > thingies. Just like as discussed in, and similar to [1]. > > Just throwing it out there, maybe we really want to avoid the directmap > (keep it unmapped) and maintain a per-mm mapping for a bunch of folios > that can be easily removed when required by guest_memfd (ftruncate, > conversion private->shared) on request. I remember talking to someone at some point about whether we could reuse the proc-local stuff for guest memory, but I cannot remember the outcome of that discussion... (or maybe I just wanted to have a discussion about it, but forgot to follow up on that thought?). I guess we wouldn't use proc-local _allocations_, but rather just set up proc-local mappings of the gmem allocations that have been removed from the direct map. I'm wondering, where exactly would be the differences to Sean's idea about messing with the CR3 register inside KVM to temporarily install page tables that contain all the gmem stuff, conceptually? Wouldn't we run into the same interrupt problems that Sean foresaw for the CR3 stuff? (which, admittedly, I still don't quite follow what these are :( ). (I've cc'd Fares Mehanna as well) > [1] https://lore.kernel.org/all/20240911143421.85612-1-faresx@amazon.de/T/#u > > -- > Cheers, > > David / dhildenb > Best, Patrick
On 12.11.24 15:40, Patrick Roy wrote: > > Hi David, > > sorry for the late response, I ended up catching the flu last week and > was out of commission for a while :( > > On Mon, 2024-11-04 at 21:30 +0000, David Hildenbrand wrote: >>>> We talked about shared (faultable) vs. private (unfaultable), and how it >>>> would interact with the directmap patches here. >>>> >>>> As discussed, having private (unfaultable) memory with the direct-map >>>> removed and shared (faultable) memory with the direct-mapping can make >>>> sense for non-TDX/AMD-SEV/... non-CoCo use cases. Not sure about CoCo, >>>> the discussion here seems to indicate that it might currently not be >>>> required. >>>> >>>> So one thing we could do is that shared (faultable) will have a direct >>>> mapping and be gup-able and private (unfaultable) memory will not have a >>>> direct mapping and is, by design, not gup-able.> >>>> Maybe it could make sense to not have a direct map for all guest_memfd >>>> memory, making it behave like secretmem (and it would be easy to >>>> implement)? But I'm not sure if that is really desirable in VM context. >>> >>> This would work for us (in this scenario, the swiotlb areas would be >>> "traditional" memory, e.g. set to shared via mem attributes instead of >>> "shared" inside KVM), it's kinda what I had prototyped in my v1 of this >>> series (well, we'd need to figure out how to get the mappings of gmem >>> back into KVM, since in this setup, short-circuiting it into >>> userspace_addr wouldn't work, unless we banish swiotlb into a different >>> memslot altogether somehow). >> >> Right. > > "right" as in, "yes we could do that"? :p "right" as in "I see how that could work" :) [...] > > I remember talking to someone at some point about whether we could reuse > the proc-local stuff for guest memory, but I cannot remember the outcome > of that discussion... (or maybe I just wanted to have a discussion about > it, but forgot to follow up on that thought?). I guess we wouldn't use > proc-local _allocations_, but rather just set up proc-local mappings of > the gmem allocations that have been removed from the direct map. Yes. And likely only for memory we really access / try access, if possible. > > I'm wondering, where exactly would be the differences to Sean's idea > about messing with the CR3 register inside KVM to temporarily install > page tables that contain all the gmem stuff, conceptually? Wouldn't we > run into the same interrupt problems that Sean foresaw for the CR3 > stuff? (which, admittedly, I still don't quite follow what these are :( > ). I'd need some more details on that. If anything would rely on the direct mapping (from IRQ context?) than ... we obviously cannot remove the direct mapping :)
On 2024-11-08 at 10:36, Elena Reshetova wrote: > On 2024-11-06 at 17:04, Derek Manwaring wrote: > > On 2024-11-04 at 08:33+0000, Elena Reshetova wrote: > > > This statement *is* for integrity section. We have a separate TDX guidance > > > on side-channels (including speculative) [3] and some speculative attacks > > > that affect confidentiality (for example spectre v1) are listed as not covered > > > by TDX but remaining SW responsibility (as they are now). > > > > Thanks for the additional info, Elena. Given that clarification, I > > definitely see direct map removal and TDX as complementary. > > Jus to clarify to make sure my comment is not misunderstood. > What I meant is that we cannot generally assume that confidentiality > leaks from CoCo guests to host/VMM via speculative channels > are completely impossible. Spectre V1 is a prime example of such a > possible leak. Dave also elaborated on other potential vectors earlier. > > The usefulness of direct map removal for CoCo guests as a concrete > mitigation for certain types of memory attacks must be precisely > evaluated per each attack vector, attack vector direction (host -> guest, > guest ->host, etc) and per each countermeasure that CoCo vendors > provide for each such case. I don't know if there is any existing study > that examines this for major CoCo vendors. I think this is what must > be done for this work in order to have a strong reasoning for its usefulness. Thanks, that makes sense. I'm a little hyperfocused on guest->host which is the opposite direction of what is generally used for the CoCo threat model. I think what both cases care about though is guest->guest. For me, guest->host matters because it's a route for guest->guest (at least in the non-CoCo world). There's some good discussion on this in David's series on attack vector controls [1]. Like you mention, beyond direction it matters which CoCo countermeasures are at play and how far they go during transient execution. That part is not clear to me for the host->guest direction involving the direct map, but agree any countermeasures like direct map removal should be evaluated based on a better understanding of what those existing countermeasures already cover and what attack is intended to be mitigated. Derek [1] https://lore.kernel.org/lkml/LV3PR12MB92658EA6CCF18F90DAAA168394532@LV3PR12MB9265.namprd12.prod.outlook.com/
On Tue, 2024-11-12 at 14:52 +0000, David Hildenbrand wrote: > On 12.11.24 15:40, Patrick Roy wrote: >> I remember talking to someone at some point about whether we could reuse >> the proc-local stuff for guest memory, but I cannot remember the outcome >> of that discussion... (or maybe I just wanted to have a discussion about >> it, but forgot to follow up on that thought?). I guess we wouldn't use >> proc-local _allocations_, but rather just set up proc-local mappings of >> the gmem allocations that have been removed from the direct map. > > Yes. And likely only for memory we really access / try access, if possible. Well, if we start on-demand mm-local mapping the things we want to access, we're back in TLB flush hell, no? And we can't know ahead-of-time what needs to be mapped, so everything would need to be mapped (unless we do something like mm-local mapping a page on first access, and then just never unmapping it again, under the assumption that establishing the mapping won't be expensive) >> >> I'm wondering, where exactly would be the differences to Sean's idea >> about messing with the CR3 register inside KVM to temporarily install >> page tables that contain all the gmem stuff, conceptually? Wouldn't we >> run into the same interrupt problems that Sean foresaw for the CR3 >> stuff? (which, admittedly, I still don't quite follow what these are :( >> ). > > I'd need some more details on that. If anything would rely on the direct > mapping (from IRQ context?) than ... we obviously cannot remove the > direct mapping :) I've talked to Fares internally, and it seems that generally doing mm-local mappings of guest memory would work for us. We also figured out what the "interrupt problem" is, namely that if we receive an interrupt while executing in a context that has mm-local mappings available, those mappings will continue to be available while the interrupt is being handled. I'm talking to my security folks to see how much of a concern this is for the speculation hardening we're trying to achieve. Will keep you in the loop there :) > -- > Cheers, > > David / dhildenb > Best, Patrick
On 15.11.24 17:59, Patrick Roy wrote: > > > On Tue, 2024-11-12 at 14:52 +0000, David Hildenbrand wrote: >> On 12.11.24 15:40, Patrick Roy wrote: >>> I remember talking to someone at some point about whether we could reuse >>> the proc-local stuff for guest memory, but I cannot remember the outcome >>> of that discussion... (or maybe I just wanted to have a discussion about >>> it, but forgot to follow up on that thought?). I guess we wouldn't use >>> proc-local _allocations_, but rather just set up proc-local mappings of >>> the gmem allocations that have been removed from the direct map. >> >> Yes. And likely only for memory we really access / try access, if possible. > > Well, if we start on-demand mm-local mapping the things we want to > access, we're back in TLB flush hell, no? At least the on-demand mapping shouldn't require a TLB flush? Only "unmapping" if we want to restrict the size of a "mapped pool" of restricted size. Anyhow, this would be a pure optimization, to avoid the expense of mapping everything, when in practice you'd like not access most of it. (my theory, happy to be told I'm wrong :) ) > And we can't know > ahead-of-time what needs to be mapped, so everything would need to be > mapped (unless we do something like mm-local mapping a page on first > access, and then just never unmapping it again, under the assumption > that establishing the mapping won't be expensive) Right, the whole problem is that we don't know that upfront. > >>> >>> I'm wondering, where exactly would be the differences to Sean's idea >>> about messing with the CR3 register inside KVM to temporarily install >>> page tables that contain all the gmem stuff, conceptually? Wouldn't we >>> run into the same interrupt problems that Sean foresaw for the CR3 >>> stuff? (which, admittedly, I still don't quite follow what these are :( >>> ). >> >> I'd need some more details on that. If anything would rely on the direct >> mapping (from IRQ context?) than ... we obviously cannot remove the >> direct mapping :) > > I've talked to Fares internally, and it seems that generally doing > mm-local mappings of guest memory would work for us. We also figured out > what the "interrupt problem" is, namely that if we receive an interrupt > while executing in a context that has mm-local mappings available, those > mappings will continue to be available while the interrupt is being > handled. Isn't that likely also the case with secretmem where we removed the directmap, but have an effective per-mm mapping in the (user-space portion) of the page table? > I'm talking to my security folks to see how much of a concern > this is for the speculation hardening we're trying to achieve. Will keep > you in the loop there :) Thanks!
On Fri, 2024-11-15 at 17:10 +0000, David Hildenbrand wrote: >> [...] >> >> I've talked to Fares internally, and it seems that generally doing >> mm-local mappings of guest memory would work for us. We also figured out >> what the "interrupt problem" is, namely that if we receive an interrupt >> while executing in a context that has mm-local mappings available, those >> mappings will continue to be available while the interrupt is being >> handled. > > Isn't that likely also the case with secretmem where we removed the > directmap, but have an effective per-mm mapping in the (user-space > portion) of the page table? Mh, that's an excellent point, I never thought of that. But with secretmem, the memory would still be protected by SMAP (admittedly, I have no idea how much this is worth in the face of all these speculative issues), right? >> I'm talking to my security folks to see how much of a concern >> this is for the speculation hardening we're trying to achieve. Will keep >> you in the loop there :) > > Thanks! > > -- > Cheers, > > David / dhildenb Best, Patrick