| Message ID | 20210818001210.4073390-2-seanjc@google.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | KVM: rseq: Fix and a test for a KVM+rseq bug | expand |
----- On Aug 17, 2021, at 8:12 PM, Sean Christopherson seanjc@google.com wrote: > Invoke rseq's NOTIFY_RESUME handler when processing the flag prior to > transferring to a KVM guest, which is roughly equivalent to an exit to > userspace and processes many of the same pending actions. While the task > cannot be in an rseq critical section as the KVM path is reachable only > via ioctl(KVM_RUN), the side effects that apply to rseq outside of a > critical section still apply, e.g. the CPU ID needs to be updated if the > task is migrated. > > Clearing TIF_NOTIFY_RESUME without informing rseq can lead to segfaults > and other badness in userspace VMMs that use rseq in combination with KVM, > e.g. due to the CPU ID being stale after task migration. I agree with the problem assessment, but I would recommend a small change to this fix. > > Fixes: 72c3c0fe54a3 ("x86/kvm: Use generic xfer to guest work function") > Reported-by: Peter Foley <pefoley@google.com> > Bisected-by: Doug Evans <dje@google.com> > Cc: Shakeel Butt <shakeelb@google.com> > Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: stable@vger.kernel.org > Signed-off-by: Sean Christopherson <seanjc@google.com> > --- > kernel/entry/kvm.c | 4 +++- > kernel/rseq.c | 4 ++-- > 2 files changed, 5 insertions(+), 3 deletions(-) > > diff --git a/kernel/entry/kvm.c b/kernel/entry/kvm.c > index 49972ee99aff..049fd06b4c3d 100644 > --- a/kernel/entry/kvm.c > +++ b/kernel/entry/kvm.c > @@ -19,8 +19,10 @@ static int xfer_to_guest_mode_work(struct kvm_vcpu *vcpu, > unsigned long ti_work) > if (ti_work & _TIF_NEED_RESCHED) > schedule(); > > - if (ti_work & _TIF_NOTIFY_RESUME) > + if (ti_work & _TIF_NOTIFY_RESUME) { > tracehook_notify_resume(NULL); > + rseq_handle_notify_resume(NULL, NULL); > + } > > ret = arch_xfer_to_guest_mode_handle_work(vcpu, ti_work); > if (ret) > diff --git a/kernel/rseq.c b/kernel/rseq.c > index 35f7bd0fced0..58c79a7918cd 100644 > --- a/kernel/rseq.c > +++ b/kernel/rseq.c > @@ -236,7 +236,7 @@ static bool in_rseq_cs(unsigned long ip, struct rseq_cs > *rseq_cs) > > static int rseq_ip_fixup(struct pt_regs *regs) > { > - unsigned long ip = instruction_pointer(regs); > + unsigned long ip = regs ? instruction_pointer(regs) : 0; > struct task_struct *t = current; > struct rseq_cs rseq_cs; > int ret; > @@ -250,7 +250,7 @@ static int rseq_ip_fixup(struct pt_regs *regs) > * If not nested over a rseq critical section, restart is useless. > * Clear the rseq_cs pointer and return. > */ > - if (!in_rseq_cs(ip, &rseq_cs)) > + if (!regs || !in_rseq_cs(ip, &rseq_cs)) I think clearing the thread's rseq_cs unconditionally here when regs is NULL is not the behavior we want when this is called from xfer_to_guest_mode_work. If we have a scenario where userspace ends up calling this ioctl(KVM_RUN) from within a rseq c.s., we really want a CONFIG_DEBUG_RSEQ=y kernel to kill this application in the rseq_syscall handler when exiting back to usermode when the ioctl eventually returns. However, clearing the thread's rseq_cs will prevent this from happening. So I would favor an approach where we simply do: if (!regs) return 0; Immediately at the beginning of rseq_ip_fixup, before getting the instruction pointer, so effectively skip all side-effects of the ip fixup code. Indeed, it is not relevant to do any fixup here, because it is nested in a ioctl system call. Effectively, this would preserve the SIGSEGV behavior when this ioctl is erroneously called by user-space from a rseq critical section. Thanks for looking into this ! Mathieu > return clear_rseq_cs(t); > ret = rseq_need_restart(t, rseq_cs.flags); > if (ret <= 0) > -- > 2.33.0.rc1.237.g0d66db33f3-goog
On Thu, Aug 19, 2021, Mathieu Desnoyers wrote: > ----- On Aug 17, 2021, at 8:12 PM, Sean Christopherson seanjc@google.com wrote: > > @@ -250,7 +250,7 @@ static int rseq_ip_fixup(struct pt_regs *regs) > > * If not nested over a rseq critical section, restart is useless. > > * Clear the rseq_cs pointer and return. > > */ > > - if (!in_rseq_cs(ip, &rseq_cs)) > > + if (!regs || !in_rseq_cs(ip, &rseq_cs)) > > I think clearing the thread's rseq_cs unconditionally here when regs is NULL > is not the behavior we want when this is called from xfer_to_guest_mode_work. > > If we have a scenario where userspace ends up calling this ioctl(KVM_RUN) > from within a rseq c.s., we really want a CONFIG_DEBUG_RSEQ=y kernel to > kill this application in the rseq_syscall handler when exiting back to usermode > when the ioctl eventually returns. > > However, clearing the thread's rseq_cs will prevent this from happening. > > So I would favor an approach where we simply do: > > if (!regs) > return 0; > > Immediately at the beginning of rseq_ip_fixup, before getting the instruction > pointer, so effectively skip all side-effects of the ip fixup code. Indeed, it > is not relevant to do any fixup here, because it is nested in a ioctl system > call. > > Effectively, this would preserve the SIGSEGV behavior when this ioctl is > erroneously called by user-space from a rseq critical section. Ha, that's effectively what I implemented first, but I changed it because of the comment in clear_rseq_cs() that says: The rseq_cs field is set to NULL on preemption or signal delivery ... as well as well as on top of code outside of the rseq assembly block. Which makes it sound like something might rely on clearing rseq_cs? Ah, or is it the case that rseq_cs is non-NULL if and only if userspace is in an rseq critical section, and because syscalls in critical sections are illegal, by definition clearing rseq_cs is a nop unless userspace is misbehaving. If that's true, what about explicitly checking that at NOTIFY_RESUME? Or is it not worth the extra code to detect an error that will likely be caught anyways? diff --git a/kernel/rseq.c b/kernel/rseq.c index 35f7bd0fced0..28b8342290b0 100644 --- a/kernel/rseq.c +++ b/kernel/rseq.c @@ -282,6 +282,13 @@ void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs) if (unlikely(t->flags & PF_EXITING)) return; + if (!regs) { +#ifdef CONFIG_DEBUG_RSEQ + if (t->rseq && rseq_get_rseq_cs(t, &rseq_cs)) + goto error; +#endif + return; + } ret = rseq_ip_fixup(regs); if (unlikely(ret < 0)) goto error; > Thanks for looking into this ! > > Mathieu > > > return clear_rseq_cs(t); > > ret = rseq_need_restart(t, rseq_cs.flags); > > if (ret <= 0) > > -- > > 2.33.0.rc1.237.g0d66db33f3-goog > > -- > Mathieu Desnoyers > EfficiOS Inc. > http://www.efficios.com
----- On Aug 19, 2021, at 7:48 PM, Sean Christopherson seanjc@google.com wrote: > On Thu, Aug 19, 2021, Mathieu Desnoyers wrote: >> ----- On Aug 17, 2021, at 8:12 PM, Sean Christopherson seanjc@google.com wrote: >> > @@ -250,7 +250,7 @@ static int rseq_ip_fixup(struct pt_regs *regs) >> > * If not nested over a rseq critical section, restart is useless. >> > * Clear the rseq_cs pointer and return. >> > */ >> > - if (!in_rseq_cs(ip, &rseq_cs)) >> > + if (!regs || !in_rseq_cs(ip, &rseq_cs)) >> >> I think clearing the thread's rseq_cs unconditionally here when regs is NULL >> is not the behavior we want when this is called from xfer_to_guest_mode_work. >> >> If we have a scenario where userspace ends up calling this ioctl(KVM_RUN) >> from within a rseq c.s., we really want a CONFIG_DEBUG_RSEQ=y kernel to >> kill this application in the rseq_syscall handler when exiting back to usermode >> when the ioctl eventually returns. >> >> However, clearing the thread's rseq_cs will prevent this from happening. >> >> So I would favor an approach where we simply do: >> >> if (!regs) >> return 0; >> >> Immediately at the beginning of rseq_ip_fixup, before getting the instruction >> pointer, so effectively skip all side-effects of the ip fixup code. Indeed, it >> is not relevant to do any fixup here, because it is nested in a ioctl system >> call. >> >> Effectively, this would preserve the SIGSEGV behavior when this ioctl is >> erroneously called by user-space from a rseq critical section. > > Ha, that's effectively what I implemented first, but I changed it because of the > comment in clear_rseq_cs() that says: > > The rseq_cs field is set to NULL on preemption or signal delivery ... as well > as well as on top of code outside of the rseq assembly block. > > Which makes it sound like something might rely on clearing rseq_cs? This comment is describing succinctly the lazy clear scheme for rseq_cs. Without the lazy clear scheme, a rseq c.s. would look like: * init(rseq_cs) * cpu = TLS->rseq::cpu_id_start * [1] TLS->rseq::rseq_cs = rseq_cs * [start_ip] ---------------------------- * [2] if (cpu != TLS->rseq::cpu_id) * goto abort_ip; * [3] <last_instruction_in_cs> * [post_commit_ip] ---------------------------- * [4] TLS->rseq::rseq_cs = NULL But as a fast-path optimization, [4] is not entirely needed because the rseq_cs descriptor contains information about the instruction pointer range of the critical section. Therefore, userspace can omit [4], but if the kernel never clears it, it means that it will have to re-read the rseq_cs descriptor's content each time it needs to check it to confirm that it is not nested over a rseq c.s.. So making the kernel lazily clear the rseq_cs pointer is just an optimization which ensures that the kernel won't do useless work the next time it needs to check rseq_cs, given that it has already validated that the userspace code is currently not within the rseq c.s. currently advertised by the rseq_cs field. > > Ah, or is it the case that rseq_cs is non-NULL if and only if userspace is in an > rseq critical section, and because syscalls in critical sections are illegal, by > definition clearing rseq_cs is a nop unless userspace is misbehaving. Not quite, as I described above. But we want it to stay set so the CONFIG_DEBUG_RSEQ code executed when returning from ioctl to userspace will be able to validate that it is not nested within a rseq critical section. > > If that's true, what about explicitly checking that at NOTIFY_RESUME? Or is it > not worth the extra code to detect an error that will likely be caught anyways? The error will indeed already be caught on return from ioctl to userspace, so I don't see any added value in duplicating this check. Thanks, Mathieu > > diff --git a/kernel/rseq.c b/kernel/rseq.c > index 35f7bd0fced0..28b8342290b0 100644 > --- a/kernel/rseq.c > +++ b/kernel/rseq.c > @@ -282,6 +282,13 @@ void __rseq_handle_notify_resume(struct ksignal *ksig, > struct pt_regs *regs) > > if (unlikely(t->flags & PF_EXITING)) > return; > + if (!regs) { > +#ifdef CONFIG_DEBUG_RSEQ > + if (t->rseq && rseq_get_rseq_cs(t, &rseq_cs)) > + goto error; > +#endif > + return; > + } > ret = rseq_ip_fixup(regs); > if (unlikely(ret < 0)) > goto error; > >> Thanks for looking into this ! >> >> Mathieu >> >> > return clear_rseq_cs(t); >> > ret = rseq_need_restart(t, rseq_cs.flags); >> > if (ret <= 0) >> > -- >> > 2.33.0.rc1.237.g0d66db33f3-goog >> >> -- >> Mathieu Desnoyers >> EfficiOS Inc. > > http://www.efficios.com
On Fri, Aug 20, 2021, Mathieu Desnoyers wrote: > Without the lazy clear scheme, a rseq c.s. would look like: > > * init(rseq_cs) > * cpu = TLS->rseq::cpu_id_start > * [1] TLS->rseq::rseq_cs = rseq_cs > * [start_ip] ---------------------------- > * [2] if (cpu != TLS->rseq::cpu_id) > * goto abort_ip; > * [3] <last_instruction_in_cs> > * [post_commit_ip] ---------------------------- > * [4] TLS->rseq::rseq_cs = NULL > > But as a fast-path optimization, [4] is not entirely needed because the rseq_cs > descriptor contains information about the instruction pointer range of the critical > section. Therefore, userspace can omit [4], but if the kernel never clears it, it > means that it will have to re-read the rseq_cs descriptor's content each time it > needs to check it to confirm that it is not nested over a rseq c.s.. > > So making the kernel lazily clear the rseq_cs pointer is just an optimization which > ensures that the kernel won't do useless work the next time it needs to check > rseq_cs, given that it has already validated that the userspace code is currently > not within the rseq c.s. currently advertised by the rseq_cs field. Thanks for the explanation, much appreciated!
On 20/08/21 20:51, Mathieu Desnoyers wrote: >> Ah, or is it the case that rseq_cs is non-NULL if and only if userspace is in an >> rseq critical section, and because syscalls in critical sections are illegal, by >> definition clearing rseq_cs is a nop unless userspace is misbehaving. > Not quite, as I described above. But we want it to stay set so the CONFIG_DEBUG_RSEQ > code executed when returning from ioctl to userspace will be able to validate that > it is not nested within a rseq critical section. > >> If that's true, what about explicitly checking that at NOTIFY_RESUME? Or is it >> not worth the extra code to detect an error that will likely be caught anyways? > The error will indeed already be caught on return from ioctl to userspace, so I > don't see any added value in duplicating this check. Sean, can you send a v2 (even for this patch only would be okay)? Thanks, Paolo
On Mon, Sep 06, 2021, Paolo Bonzini wrote: > On 20/08/21 20:51, Mathieu Desnoyers wrote: > > > Ah, or is it the case that rseq_cs is non-NULL if and only if userspace is in an > > > rseq critical section, and because syscalls in critical sections are illegal, by > > > definition clearing rseq_cs is a nop unless userspace is misbehaving. > > Not quite, as I described above. But we want it to stay set so the CONFIG_DEBUG_RSEQ > > code executed when returning from ioctl to userspace will be able to validate that > > it is not nested within a rseq critical section. > > > > > If that's true, what about explicitly checking that at NOTIFY_RESUME? Or is it > > > not worth the extra code to detect an error that will likely be caught anyways? > > The error will indeed already be caught on return from ioctl to userspace, so I > > don't see any added value in duplicating this check. > > Sean, can you send a v2 (even for this patch only would be okay)? Made it all the way to v3 while you were out :-) https://lkml.kernel.org/r/20210901203030.1292304-1-seanjc@google.com
diff --git a/kernel/entry/kvm.c b/kernel/entry/kvm.c index 49972ee99aff..049fd06b4c3d 100644 --- a/kernel/entry/kvm.c +++ b/kernel/entry/kvm.c @@ -19,8 +19,10 @@ static int xfer_to_guest_mode_work(struct kvm_vcpu *vcpu, unsigned long ti_work) if (ti_work & _TIF_NEED_RESCHED) schedule(); - if (ti_work & _TIF_NOTIFY_RESUME) + if (ti_work & _TIF_NOTIFY_RESUME) { tracehook_notify_resume(NULL); + rseq_handle_notify_resume(NULL, NULL); + } ret = arch_xfer_to_guest_mode_handle_work(vcpu, ti_work); if (ret) diff --git a/kernel/rseq.c b/kernel/rseq.c index 35f7bd0fced0..58c79a7918cd 100644 --- a/kernel/rseq.c +++ b/kernel/rseq.c @@ -236,7 +236,7 @@ static bool in_rseq_cs(unsigned long ip, struct rseq_cs *rseq_cs) static int rseq_ip_fixup(struct pt_regs *regs) { - unsigned long ip = instruction_pointer(regs); + unsigned long ip = regs ? instruction_pointer(regs) : 0; struct task_struct *t = current; struct rseq_cs rseq_cs; int ret; @@ -250,7 +250,7 @@ static int rseq_ip_fixup(struct pt_regs *regs) * If not nested over a rseq critical section, restart is useless. * Clear the rseq_cs pointer and return. */ - if (!in_rseq_cs(ip, &rseq_cs)) + if (!regs || !in_rseq_cs(ip, &rseq_cs)) return clear_rseq_cs(t); ret = rseq_need_restart(t, rseq_cs.flags); if (ret <= 0)
Invoke rseq's NOTIFY_RESUME handler when processing the flag prior to transferring to a KVM guest, which is roughly equivalent to an exit to userspace and processes many of the same pending actions. While the task cannot be in an rseq critical section as the KVM path is reachable only via ioctl(KVM_RUN), the side effects that apply to rseq outside of a critical section still apply, e.g. the CPU ID needs to be updated if the task is migrated. Clearing TIF_NOTIFY_RESUME without informing rseq can lead to segfaults and other badness in userspace VMMs that use rseq in combination with KVM, e.g. due to the CPU ID being stale after task migration. Fixes: 72c3c0fe54a3 ("x86/kvm: Use generic xfer to guest work function") Reported-by: Peter Foley <pefoley@google.com> Bisected-by: Doug Evans <dje@google.com> Cc: Shakeel Butt <shakeelb@google.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: stable@vger.kernel.org Signed-off-by: Sean Christopherson <seanjc@google.com> --- kernel/entry/kvm.c | 4 +++- kernel/rseq.c | 4 ++-- 2 files changed, 5 insertions(+), 3 deletions(-)