| Message ID | 20180724065759.19186-17-takahiro.akashi@linaro.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | arm64: kexec: add kexec_file_load() support | expand |
Hi Akashi, On 24/07/18 07:57, AKASHI Takahiro wrote: > Adding "kaslr-seed" to dtb enables triggering kaslr, or kernel virtual > address randomization, at secondary kernel boot. Hmm, there are three things that get moved by CONFIG_RANDOMIZE_BASE. The kernel physical placement when booted via the EFIstub, the kernel-text VAs and the location of memory in the linear-map region. Adding the kaslr-seed only does the last two. This means the physical placement of the new kernel is predictable from /proc/iomem ... but this also tells you the physical placement of the current kernel, so I don't think this is a problem. > We always do this as it will have no harm on kaslr-incapable kernel. > We don't have any "switch" to turn off this feature directly, but still > can suppress it by passing "nokaslr" as a kernel boot argument. > diff --git a/arch/arm64/kernel/machine_kexec_file.c b/arch/arm64/kernel/machine_kexec_file.c > index 7356da5a53d5..47a4fbd0dc34 100644 > --- a/arch/arm64/kernel/machine_kexec_file.c > +++ b/arch/arm64/kernel/machine_kexec_file.c > @@ -158,6 +160,12 @@ static int setup_dtb(struct kimage *image, Don't you need to reserve some space in the area you vmalloc()d for the DT? > + /* add kaslr-seed */ > + get_random_bytes(&value, sizeof(value)); What happens if the crng isn't ready? It looks like this will print a warning that these random-bytes aren't really up to standard, but the new kernel doesn't know this happened. crng_ready() isn't exposed, all we could do now is wait_for_random_bytes(), but that may wait forever because we do this unconditionally. I'd prefer to leave this feature until we can check crng_ready(), and skip adding a dodgy-seed if its not-ready. This avoids polluting the next-kernel's entropy pool. > + ret = fdt_setprop(buf, nodeoffset, "kaslr-seed", &value, sizeof(value)); Nit: It would be nice if this string were in a header file somewhere, to void future refactoring typos. Thanks, James
On Thu, Jul 26, 2018 at 02:40:49PM +0100, James Morse wrote: > Hi Akashi, > > On 24/07/18 07:57, AKASHI Takahiro wrote: > > Adding "kaslr-seed" to dtb enables triggering kaslr, or kernel virtual > > address randomization, at secondary kernel boot. > > Hmm, there are three things that get moved by CONFIG_RANDOMIZE_BASE. The kernel > physical placement when booted via the EFIstub, the kernel-text VAs and the > location of memory in the linear-map region. Adding the kaslr-seed only does the > last two. Yes, but I think that I and Mark has agreed that "kaslr" meant "virtual" randomisation, not including "physical" randomisation. > This means the physical placement of the new kernel is predictable from > /proc/iomem ... but this also tells you the physical placement of the current > kernel, so I don't think this is a problem. > > > > We always do this as it will have no harm on kaslr-incapable kernel. > > > We don't have any "switch" to turn off this feature directly, but still > > can suppress it by passing "nokaslr" as a kernel boot argument. > > > > diff --git a/arch/arm64/kernel/machine_kexec_file.c b/arch/arm64/kernel/machine_kexec_file.c > > index 7356da5a53d5..47a4fbd0dc34 100644 > > --- a/arch/arm64/kernel/machine_kexec_file.c > > +++ b/arch/arm64/kernel/machine_kexec_file.c > > @@ -158,6 +160,12 @@ static int setup_dtb(struct kimage *image, > > Don't you need to reserve some space in the area you vmalloc()d for the DT? No, I don't think so. All the data to be loaded are temporarily saved in kexec buffers, which will eventually be copied to target locations in machine_kexec (arm64_relocate_new_kernel, which, unlike its name, will handle not only kernel but also other data as well). > > > + /* add kaslr-seed */ > > + get_random_bytes(&value, sizeof(value)); > > What happens if the crng isn't ready? > > It looks like this will print a warning that these random-bytes aren't really up > to standard, but the new kernel doesn't know this happened. > > crng_ready() isn't exposed, all we could do now is > wait_for_random_bytes(), but that may wait forever because we do this > unconditionally. > > I'd prefer to leave this feature until we can check crng_ready(), and skip > adding a dodgy-seed if its not-ready. This avoids polluting the next-kernel's > entropy pool. OK. I would try to follow the same way as Bhupesh's userspace patch does for kaslr-seed: http://lists.infradead.org/pipermail/kexec/2018-April/020564.html if (not found kaslr-seed in 1st kernel's dtb) don't care; go ahead else if (current kaslr-seed != 0) error if (crng_ready()) ; FIXME, it's a local macro get_random_bytes(non-blocking) set new kaslr-seed else error > > > + ret = fdt_setprop(buf, nodeoffset, "kaslr-seed", &value, sizeof(value)); > > Nit: It would be nice if this string were in a header file somewhere, to void > future refactoring typos. OK. (but in this file for now as I mentioned in my previous reply) Thanks, -Takahiro AKASHI > > Thanks, > > James
On 27 July 2018 at 11:22, James Morse <james.morse@arm.com> wrote: > Hi Akashi, > > > On 07/27/2018 09:31 AM, AKASHI Takahiro wrote: > > On Thu, Jul 26, 2018 at 02:40:49PM +0100, James Morse wrote: > > On 24/07/18 07:57, AKASHI Takahiro wrote: > > Adding "kaslr-seed" to dtb enables triggering kaslr, or kernel virtual > address randomization, at secondary kernel boot. > > Hmm, there are three things that get moved by CONFIG_RANDOMIZE_BASE. The > kernel > physical placement when booted via the EFIstub, the kernel-text VAs and the > location of memory in the linear-map region. Adding the kaslr-seed only does > the > last two. > > Yes, but I think that I and Mark has agreed that "kaslr" meant > "virtual" randomisation, not including "physical" randomisation. > > Okay, I'll update my terminology! > > > This means the physical placement of the new kernel is predictable from > /proc/iomem ... but this also tells you the physical placement of the > current > kernel, so I don't think this is a problem. > > > We always do this as it will have no harm on kaslr-incapable kernel. > > We don't have any "switch" to turn off this feature directly, but still > can suppress it by passing "nokaslr" as a kernel boot argument. > > diff --git a/arch/arm64/kernel/machine_kexec_file.c > b/arch/arm64/kernel/machine_kexec_file.c > index 7356da5a53d5..47a4fbd0dc34 100644 > --- a/arch/arm64/kernel/machine_kexec_file.c > +++ b/arch/arm64/kernel/machine_kexec_file.c > @@ -158,6 +160,12 @@ static int setup_dtb(struct kimage *image, > > Don't you need to reserve some space in the area you vmalloc()d for the DT? > > No, I don't think so. > All the data to be loaded are temporarily saved in kexec buffers, > which will eventually be copied to target locations in machine_kexec > (arm64_relocate_new_kernel, which, unlike its name, will handle > not only kernel but also other data as well). > > > I think we're speaking at cross purposes. Don't you need: > > | buf_size += fdt_prop_len("kaslr―seed", sizeof(u64)); > > > You can't assume the existing DTB had a kaslr-seed property, and the > difference may take us over a PAGE_SIZE boundary. > > > > > + /* add kaslr-seed */ > + get_random_bytes(&value, sizeof(value)); > > What happens if the crng isn't ready? > > It looks like this will print a warning that these random-bytes aren't > really up > to standard, but the new kernel doesn't know this happened. > > crng_ready() isn't exposed, all we could do now is > wait_for_random_bytes(), but that may wait forever because we do this > unconditionally. > > I'd prefer to leave this feature until we can check crng_ready(), and skip > adding a dodgy-seed if its not-ready. This avoids polluting the > next-kernel's > entropy pool. > > OK. I would try to follow the same way as Bhupesh's userspace patch > does for kaslr-seed: > http://lists.infradead.org/pipermail/kexec/2018-April/020564.html > > > (I really don't understand this 'copying code from user-space' that happens > with kexec_file_load) > > > if (not found kaslr-seed in 1st kernel's dtb) > don't care; go ahead > > > Don' t bother. As you say in the commit-message its harmless if the new > kernel doesn't support it. > Always having this would let you use kexec_file_load as a bootloader that > can get the crng to > provide decent entropy even if the platform bootloader can't. > > > else > if (current kaslr-seed != 0) > error > > > Don't bother. If this happens its a bug in another part of the kernel that > doesn't affect this one. We aren't second-guessing the file-system when we > read the kernel-fd, lets keep this simple. > > if (crng_ready()) ; FIXME, it's a local macro > get_random_bytes(non-blocking) > set new kaslr-seed > else > error > > error? Something like pr_warn_once(). > > I thought the kaslr-seed was added to the entropy pool, but now I look again > I see its a separate EFI table. So the new kernel will add the same entropy > ... that doesn't sound clever. (I can't see where its zero'd or > re-initialised) > We do have a hook for that: grep for update_efi_random_seed()
James, All the changes mentioned below were applied to my coming v13. On Fri, Jul 27, 2018 at 10:22:31AM +0100, James Morse wrote: > Hi Akashi, > > > On 07/27/2018 09:31 AM, AKASHI Takahiro wrote: > >On Thu, Jul 26, 2018 at 02:40:49PM +0100, James Morse wrote: > >>On 24/07/18 07:57, AKASHI Takahiro wrote: > >>>Adding "kaslr-seed" to dtb enables triggering kaslr, or kernel virtual > >>>address randomization, at secondary kernel boot. > >>Hmm, there are three things that get moved by CONFIG_RANDOMIZE_BASE. The kernel > >>physical placement when booted via the EFIstub, the kernel-text VAs and the > >>location of memory in the linear-map region. Adding the kaslr-seed only does the > >>last two. > >Yes, but I think that I and Mark has agreed that "kaslr" meant > >"virtual" randomisation, not including "physical" randomisation. > Okay, I'll update my terminology! > > > >>This means the physical placement of the new kernel is predictable from > >>/proc/iomem ... but this also tells you the physical placement of the current > >>kernel, so I don't think this is a problem. > >> > >> > >>>We always do this as it will have no harm on kaslr-incapable kernel. > >>>We don't have any "switch" to turn off this feature directly, but still > >>>can suppress it by passing "nokaslr" as a kernel boot argument. > >> > >>>diff --git a/arch/arm64/kernel/machine_kexec_file.c b/arch/arm64/kernel/machine_kexec_file.c > >>>index 7356da5a53d5..47a4fbd0dc34 100644 > >>>--- a/arch/arm64/kernel/machine_kexec_file.c > >>>+++ b/arch/arm64/kernel/machine_kexec_file.c > >>>@@ -158,6 +160,12 @@ static int setup_dtb(struct kimage *image, > >>Don't you need to reserve some space in the area you vmalloc()d for the DT? > >No, I don't think so. > >All the data to be loaded are temporarily saved in kexec buffers, > >which will eventually be copied to target locations in machine_kexec > >(arm64_relocate_new_kernel, which, unlike its name, will handle > >not only kernel but also other data as well). > > I think we're speaking at cross purposes. Don't you need: > > | buf_size += fdt_prop_len("kaslr―seed", sizeof(u64)); > > > You can't assume the existing DTB had a kaslr-seed property, and the > difference may take us over a PAGE_SIZE boundary. I see, I will add that. > > > > >> > >>>+ /* add kaslr-seed */ > >>>+ get_random_bytes(&value, sizeof(value)); > >>What happens if the crng isn't ready? > >> > >>It looks like this will print a warning that these random-bytes aren't really up > >>to standard, but the new kernel doesn't know this happened. > >> > >>crng_ready() isn't exposed, all we could do now is > >>wait_for_random_bytes(), but that may wait forever because we do this > >>unconditionally. > >> > >>I'd prefer to leave this feature until we can check crng_ready(), and skip > >>adding a dodgy-seed if its not-ready. This avoids polluting the next-kernel's > >>entropy pool. > >OK. I would try to follow the same way as Bhupesh's userspace patch > >does for kaslr-seed: > >http://lists.infradead.org/pipermail/kexec/2018-April/020564.html > > (I really don't understand this 'copying code from user-space' that happens > with kexec_file_load) > > > > if (not found kaslr-seed in 1st kernel's dtb) > > don't care; go ahead > > Don' t bother. As you say in the commit-message its harmless if the new > kernel doesn't support it. > Always having this would let you use kexec_file_load as a bootloader that > can get the crng to > provide decent entropy even if the platform bootloader can't. OK, but anyway previous "kaslr-seed" will be dropped first. > > > else > > if (current kaslr-seed != 0) > > error > > Don't bother. If this happens its a bug in another part of the kernel that > doesn't affect this one. We aren't second-guessing the file-system when we > read the kernel-fd, lets keep this simple. OK > > if (crng_ready()) ; FIXME, it's a local macro > > get_random_bytes(non-blocking) > > set new kaslr-seed > > else > > error > error? Something like pr_warn_once(). It was changed to pr_notice() since there is nothing wrong. Thanks, -Takahiro AKASHI > I thought the kaslr-seed was added to the entropy pool, but now I look again > I see its a separate EFI table. So the new kernel will add the same entropy > ... that doesn't sound clever. (I can't see where its zero'd or > re-initialised) > > > > Thanks, > > James
diff --git a/arch/arm64/kernel/machine_kexec_file.c b/arch/arm64/kernel/machine_kexec_file.c index 7356da5a53d5..47a4fbd0dc34 100644 --- a/arch/arm64/kernel/machine_kexec_file.c +++ b/arch/arm64/kernel/machine_kexec_file.c @@ -16,6 +16,7 @@ #include <linux/libfdt.h> #include <linux/memblock.h> #include <linux/of_fdt.h> +#include <linux/random.h> #include <linux/slab.h> #include <linux/types.h> #include <linux/vmalloc.h> @@ -46,6 +47,7 @@ static int setup_dtb(struct kimage *image, void *buf = NULL; size_t buf_size, range_size; int nodeoffset; + u64 value; int ret; /* check ranges against root's #address-cells and #size-cells */ @@ -158,6 +160,12 @@ static int setup_dtb(struct kimage *image, } } + /* add kaslr-seed */ + get_random_bytes(&value, sizeof(value)); + ret = fdt_setprop(buf, nodeoffset, "kaslr-seed", &value, sizeof(value)); + if (ret) + goto out_err; + /* trim a buffer */ fdt_pack(buf); *dtb_buf = buf;
Adding "kaslr-seed" to dtb enables triggering kaslr, or kernel virtual address randomization, at secondary kernel boot. We always do this as it will have no harm on kaslr-incapable kernel. We don't have any "switch" to turn off this feature directly, but still can suppress it by passing "nokaslr" as a kernel boot argument. Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Will Deacon <will.deacon@arm.com> --- arch/arm64/kernel/machine_kexec_file.c | 8 ++++++++ 1 file changed, 8 insertions(+)