linux-kernel - Re: [PATCH v2 3/3] x86/fpu/xstate: Invalidate fpregs when __fpu_restore

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Date:   Wed, 18 Dec 2019 16:54:49 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Rik van Riel <riel@...riel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 3/3] x86/fpu/xstate: Invalidate fpregs when
 __fpu_restore_sig() fails

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   FPU registers are no longer valid for this task.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.

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?

Sebastian