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SubjectRe: [PATCH v2 3/3] x86/fpu/xstate: Invalidate fpregs when __fpu_restore_sig() fails
On 2019-12-18 12:53:59 [-0800], Yu-cheng Yu wrote:
> 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.

The context switch may only happen after fpregs_unlock().

> However, at this point, fpu_fpregs_owner_ctx has not been changed; it could
> still be another task's FPU.

TIF_NEED_FPU_LOAD is set for the task in __fpu__restore_sig() and its
context (__fpu_invalidate_fpregs_state()) has been invalidated. So the
FPU register may contain another task's content and
fpu_fpregs_owner_ctx points to another context.

> 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.

So far everything is legal. However. If there is a context switch before
fpregs_lock() then this is bad before we don't account for that.
So that:

diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -352,6 +352,7 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
return 0;
+ fpregs_deactivate(fpu);

@@ -403,6 +404,8 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
if (!ret)
+ else
+ fpregs_deactivate(fpu);


Should be enough.

> > 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.

Okay. Can you please try the above and if not, I try that glibc thing myself.

> Thanks,
> Yu-cheng


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