linux-kernel - Re: [PATCH] x86 spinlock: Fix memory corruption on completing completions

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Date:	Fri, 06 Feb 2015 20:03:14 +0300
From:	Andrey Ryabinin <a.ryabinin@...sung.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Sasha Levin <sasha.levin@...cle.com>
Cc:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Peter Anvin <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Waiman Long <waiman.long@...com>,
	Dave Jones <davej@...hat.com>, Oleg Nesterov <oleg@...hat.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Jason Wang <jasowang@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>,
	virtualization <virtualization@...ts.linux-foundation.org>,
	xen-devel@...ts.xenproject.org, Rik van Riel <riel@...hat.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] x86 spinlock: Fix memory corruption on completing
 completions

On 02/06/2015 07:15 PM, Linus Torvalds wrote:
> On Fri, Feb 6, 2015 at 7:20 AM, Sasha Levin <sasha.levin@...cle.com> wrote:
>>
>> Can we modify it slightly to avoid potentially accessing invalid memory:
> 
> So I think there's a race with that.
> 
> And I'll warn you: the kernel does do speculative reads of memory that
> might be invalid, not just in places like this. See the comment in
> get_user_huge_page() for example, where we knowingly do speculative
> reads, but hide it if DEBUG_PAGEALLOC is set.
> 
> More commonly, CONFIG_DCACHE_WORD_ACCESS is very much about doing
> speculative reads. Now, that access is hidden inside an asm, so KASan
> won't see it, but there might well be others.
> 
> You probably don't see them very much just because they are so rarely
> a problem, and most of the time it's not to other processes stack but
> to allocated structures where freeing takes long enough to basically
> hide any small race..
> 
> In other words: I suspect it would be good to instead just teach KASan
> about "this is a speculative read" and just suppress the warning for
> those instead.
> 

We can suppress warnings by wrapping such speculative reads with
kasan_disable_current()/kasan_enable_current() calls.

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