Messages in this thread | | | Subject | Re: [PATCH v2 3/3] x86/fpu/xstate: Invalidate fpregs when __fpu_restore_sig() fails | From | Yu-cheng Yu <> | Date | Wed, 18 Dec 2019 12:53:59 -0800 |
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On Wed, 2019-12-18 at 16:54 +0100, Sebastian Andrzej Siewior wrote: > On 2019-12-12 13:08:55 [-0800], Yu-cheng Yu wrote: > > In __fpu_restore_sig(),'init_fpstate.xsave' and part of 'fpu->state.xsave' > > are restored separately to xregs. However, as stated in __cpu_invalidate_ > > fpregs_state(), > > > > Any code that clobbers the FPU registers or updates the in-memory > > FPU state for a task MUST let the rest of the kernel know that the > > FPU registers are no longer valid for this task. > > > > and this code violates that rule. Should the restoration fail, the other > > task's context is corrupted. > > > > This problem does not occur very often because copy_*_to_xregs() succeeds > > most of the time. > > why "most of the time"? It should always succeed. We talk here about > __fpu__restore_sig() correct? Using init_fpstate as part of restore > process isn't the "default" case. If the restore _here_ fails then it > fails. > > > It occurs, for instance, in copy_user_to_fpregs_ > > zeroing() when the first half of the restoration succeeds and the other > > half fails. This can be triggered by running glibc tests, where a non- > > present user stack page causes the XRSTOR to fail. > > So if copy_user_to_fpregs_zeroing() fails then we go to the slowpath. > Then we load the FPU register with copy_kernel_to_xregs_err(). > In the end they are either enabled (fpregs_mark_activate()) or cleared > if it failed (fpu__clear()). Don't see here a problem.
I could have explained this better, sorry! I will explain the first case below; other cases are similar.
In copy_user_to_fpregs_zeroing(), we have:
if (user_xsave()) { ... if (unlikely(init_bv)) copy_kernel_to_xregs(&init_fpstate.xsave, init_bv); return copy_user_to_xregs(buf, xbv); ... }
The copy_user_to_xregs() may fail, and when that happens, before going to the slow path, there is fpregs_unlock() and context switches may happen. However, at this point, fpu_fpregs_owner_ctx has not been changed; it could still be another task's FPU. For this to happen and to be detected, the user stack page needs to be non-present, fpu_fpregs_owner_ctx need to be another task, and that other task needs to be able to detect its registers are modified. The last factor is not easy to reproduce, and a CET control-protection fault helps.
> > Can you tell me which glibc test? I would like to reproduce this. > > > The introduction of supervisor xstates and CET, while not contributing to > > the problem, makes it more detectable. After init_fpstate and the Shadow > > Stack pointer have been restored to xregs, the XRSTOR from user stack > > fails and fpu_fpregs_owner_ctx is not updated. The task currently owning > > fpregs then uses the corrupted Shadow Stack pointer and triggers a control- > > protection fault. > > So I don't need new HW with supervisor and CET? A plain KVM box with > SSE2 and so should be enough?
What I do is, clone the whole glibc source, and run mutiple copies of "make check". In about 40 minutes or so, there are unexplained seg faults, or a few control-protection faults (if you enable CET). Please let me know if more clarification is needed.
Thanks, Yu-cheng
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