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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
	"H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Tony Luck <tony.luck@intel.com>,
	Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
	Rik van Riel <riel@surriel.com>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Fenghua Yu <fenghua.yu@intel.com>,
	Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v2 3/3] x86/fpu/xstate: Invalidate fpregs when __fpu_restore_sig() fails
Date: Wed, 18 Dec 2019 16:54:49 +0100
Message-ID: <20191218155449.sk4gjabtynh67jqb@linutronix.de> (raw)
In-Reply-To: <20191212210855.19260-4-yu-cheng.yu@intel.com>

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   FPU registers are no longer valid for this task.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

  reply index

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-12 21:08 [PATCH v2 0/3] Fix small issues in XSAVES Yu-cheng Yu
2019-12-12 21:08 ` [PATCH v2 1/3] x86/fpu/xstate: Fix small issues before adding supervisor xstates Yu-cheng Yu
2019-12-20 20:19   ` Sebastian Andrzej Siewior
2020-01-06 18:15   ` [tip: x86/fpu] x86/fpu/xstate: Fix small issues tip-bot2 for Yu-cheng Yu
2019-12-12 21:08 ` [PATCH v2 2/3] x86/fpu/xstate: Make xfeature_is_supervisor()/xfeature_is_user() return bool Yu-cheng Yu
2019-12-20 20:19   ` Sebastian Andrzej Siewior
2019-12-20 20:33     ` Yu-cheng Yu
2020-01-06 18:15   ` [tip: x86/fpu] " tip-bot2 for Yu-cheng Yu
2019-12-12 21:08 ` [PATCH v2 3/3] x86/fpu/xstate: Invalidate fpregs when __fpu_restore_sig() fails Yu-cheng Yu
2019-12-18 15:54   ` Sebastian Andrzej Siewior [this message]
2019-12-18 20:53     ` Yu-cheng Yu
2019-12-19 14:22       ` Sebastian Andrzej Siewior
2019-12-19 16:44         ` Yu-cheng Yu
2019-12-19 17:16           ` Sebastian Andrzej Siewior
2019-12-19 17:40             ` Yu-cheng Yu
2019-12-20 19:59               ` [PATCH] x86/fpu: Deacticate FPU state after failure during state load Sebastian Andrzej Siewior
2020-01-07 12:52                 ` [tip: x86/fpu] x86/fpu: Deactivate " tip-bot2 for Sebastian Andrzej Siewior
2020-01-07 20:41                   ` Andy Lutomirski
2020-01-07 20:38                     ` Yu-cheng Yu
2020-01-07 21:11                     ` Sebastian Andrzej Siewior
2020-01-08 11:45                       ` Borislav Petkov
2020-01-08 11:46                     ` Borislav Petkov
2019-12-20 20:16               ` [PATCH v2 3/3] x86/fpu/xstate: Invalidate fpregs when __fpu_restore_sig() fails Sebastian Andrzej Siewior
2019-12-20 20:32                 ` Yu-cheng Yu
  -- strict thread matches above, loose matches on Subject: below --
2019-12-05 18:26 [PATCH " Yu-cheng Yu
2019-12-07  4:38 ` [PATCH v2 " Yu-cheng Yu

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