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SubjectRe: [PATCH v2 3/3] x86/fpu/xstate: Invalidate fpregs when __fpu_restore_sig() fails
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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 FPU registers are no longer valid for this task.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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