Re: [Xen-devel] [PATCH] x86 spinlock: Fix memory corruption on completing completions

[Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx>]

On 02/06/2015 07:15 PM, Linus Torvalds wrote:
> On Fri, Feb 6, 2015 at 7:20 AM, Sasha Levin <sasha.levin@xxxxxxxxxx> 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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