linux-kernel - Re: [PATCH 3/3] x86, fpu: fix math_state_restore() race with kernel_fpu

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Message-ID: <54B877BC.3070905@redhat.com>
Date:	Thu, 15 Jan 2015 21:30:20 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Suresh Siddha <sbsiddha@...il.com>
CC:	linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
	matt.fleming@...el.com, bp@...e.de, pbonzini@...hat.com,
	tglx@...utronix.de, luto@...capital.net
Subject: Re: [PATCH 3/3] x86, fpu: fix math_state_restore() race with kernel_fpu_begin()

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On 01/15/2015 02:20 PM, Oleg Nesterov wrote:
> math_state_restore() can race with kernel_fpu_begin() if irq comes 
> right after __thread_fpu_begin(), __save_init_fpu() will overwrite 
> fpu->state we are going to restore.
> 
> Add 2 simple helpers, kernel_fpu_disable() and kernel_fpu_enable() 
> which simply set/clear in_kernel_fpu, and change
> math_state_restore() to exclude kernel_fpu_begin() in between.
> 
> Alternatively we could use local_irq_save/restore, but probably
> these new helpers can have more users.
> 
> Perhaps they should disable/enable preemption themselves, in this
> case we can remove preempt_disable() in __restore_xstate_sig().

Given that math_state_restore does an implicit preempt_disable
through local_irq_disable, I am not sure whether adding an
explicit preempt_disable would be good or bad.

It's not like the additional locking rule makes this code any
more complex.

> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Reviewed-by: Rik van Riel <riel@...hat.com>

- -- 
All rights reversed
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