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From: David Laight <David.Laight@ACULAB.COM>
To: 'Sebastian Andrzej Siewior' <bigeasy@linutronix.de>,
	Barret Rhoden <brho@google.com>
Cc: Borislav Petkov <bp@alien8.de>,
	Josh Bleecher Snyder <josharian@gmail.com>,
	Rik van Riel <riel@surriel.com>,
	"x86@kernel.org" <x86@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Ingo Molnar" <mingo@redhat.com>, "ian@airs.com" <ian@airs.com>,
	Austin Clements <austin@google.com>,
	David Chase <drchase@golang.org>
Subject: RE: [PATCH v2] x86/fpu: Don't cache access to fpu_fpregs_owner_ctx
Date: Fri, 29 Nov 2019 16:57:42 +0000
Message-ID: <d99a4a85441d48b1b34a7fb648d12379@AcuMS.aculab.com> (raw)
In-Reply-To: <20191128085306.hxfa2o3knqtu4wfn@linutronix.de>

From Sebastian Andrzej Siewior
> Sent: 28 November 2019 08:53
> The state/owner of FPU is saved fpu_fpregs_owner_ctx by pointing to the
> context that is currently loaded. It never changed during the life time
> of a task and remained stable/constant.
> 
> Since deferred loading the FPU registers on return to userland, the
> content of fpu_fpregs_owner_ctx may change during preemption and must
> not be cached.
> This went unnoticed for some time and was now noticed, in particular
> gcc-9 is able to cache that load in copy_fpstate_to_sigframe() and reuse
> it in the retry loop:
> 
>   copy_fpstate_to_sigframe()
>     load fpu_fpregs_owner_ctx and save on stack
>     fpregs_lock()
>     copy_fpregs_to_sigframe() /* failed */
>     fpregs_unlock()
>          *** PREEMPTION, another uses FPU, changes fpu_fpregs_owner_ctx ***
> 
>     fault_in_pages_writeable() /* succeed, retry */
> 
>     fpregs_lock()
> 	__fpregs_load_activate()
> 	  fpregs_state_valid() /* uses fpu_fpregs_owner_ctx from stack */
>     copy_fpregs_to_sigframe() /* succeeds, random FPU content */

Should both fpregs_lock() and fpregs_unlock() contain a barrier() (or "memory" clobber)
do stop the compiler moving anything across them?

	David

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Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

  parent reply index

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-26 19:49 AVX register corruption from signal delivery Barret Rhoden
2019-11-26 20:20 ` Sebastian Andrzej Siewior
2019-11-26 21:23   ` Barret Rhoden
2019-11-26 22:13     ` Borislav Petkov
2019-11-26 22:30       ` Andy Lutomirski
2019-11-26 23:00         ` Borislav Petkov
2019-11-27 12:42     ` [PATCH] x86/fpu: Don't cache access to fpu_fpregs_owner_ctx Sebastian Andrzej Siewior
2019-11-27 14:07       ` Borislav Petkov
2019-11-27 18:42         ` Barret Rhoden
2019-11-28  8:53           ` [PATCH v2] " Sebastian Andrzej Siewior
2019-11-28  9:22             ` [tip: x86/urgent] " tip-bot2 for Sebastian Andrzej Siewior
2019-11-29 16:57             ` David Laight [this message]
2019-11-29 17:08               ` [PATCH v2] " 'Sebastian Andrzej Siewior'
2019-11-27 15:46       ` [PATCH] " Rik van Riel

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