linux-kernel - Re: [patches] VM-related fixes

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Message-ID: <CAMP5XgeXvrQ4_cEWGiiHzbnYrzx6Nej_O4H_bA5JzLmUtvYALQ@mail.gmail.com>
Date:	Mon, 5 Mar 2012 22:36:47 -0800
From:	Arve Hjønnevåg <arve@...roid.com>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, airlied@...ux.ie,
	carsteno@...ibm.com, steiner@....com
Subject: Re: [patches] VM-related fixes

On Mon, Mar 5, 2012 at 9:57 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Mon, Mar 05, 2012 at 09:25:05PM -0800, Arve Hj?nnev?g wrote:
>> > Sorry, no.
>> > ? ? ? ? ? ? ? ?down_write(&mm->mmap_sem);
>> > ? ? ? ? ? ? ? ?vma = proc->vma;
>> > ? ? ? ? ? ? ? ?if (vma && mm != vma->vm_mm) {
>> > does *not* do what you seem to describe; there's nothing to protect you
>> > from proc->vma getting freed under you right between load from proc->vma
>> > and check of vma->mm. ?->mmap_sem on the right mm would prevent that,
>> > but this one doesn't guarantee anything. ?Get preempted after the second
>> > line quoted above and by the time you get the timeslice back, you might
>> > have had munmap() done by another thread, with vma freed, its memory
>> > recycled, etc.
>> >
>>
>> OK, if the memory got freed and then re-used by someone who stored a
>> value that matched a pointer to the mm struct that was just locked,
>> this check will fail to catch it. I can check against a cached vm_mm
>> member from mmap instead, assuming this will not change before
>> ->close() is called. Does that sound reasonable, or is there a better
>> way to check this?
>
> Huh?  Sorry, I hadn't been able to parse that - what do you want to cache,

The vm_mm member of the vma.

> where and what do you want to check?  Again, at that point *(proc->vma)
> may very well be random garbage, so looking at it would be pointless;

Yes it may be random garbage, but we only have a problem if that
random garbage happens to match the mm pointer we just used.

> the value you had at ->mmap() time would be simply current->mm of mmap(2)
> caller; if you want to check that it matches that of opener, fine,
> but then why not do just that in ->mmap()?

If the opener calls exec does the mm returned by get_task_mm change?
If so, the mmap_sem I locked will not match the vma saved in mmap() so
I need the check above. If exec creates a new task struct then a check
in mmap would be sufficient, but I was not under the impression that
this happens.

-- 
Arve Hjønnevåg
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