Re: Multiple potential races on vma->vm_flags

[Vlastimil Babka <vbabka@suse.cz>]

On 09/11/2015 12:39 PM, Kirill A. Shutemov wrote:
> On Thu, Sep 10, 2015 at 03:27:59PM +0200, Andrey Konovalov wrote:
>> Can a vma be shared among a few mm's?
>
> Define "shared".
>
> vma can belong only to one process (mm_struct), but it can be accessed
> from other process like in rmap case below.
>
> rmap uses anon_vma_lock for anon vma and i_mmap_rwsem for file vma to make
> sure that the vma will not disappear under it.
>
>> If yes, then taking current->mm->mmap_sem to protect vma is not enough.
>
> Depends on what protection you are talking about.
>
>> In the first report below both T378 and T398 take
>> current->mm->mmap_sem at mm/mlock.c:650, but they turn out to be
>> different locks (the addresses are different).
>
> See i_mmap_lock_read() in T398. It will guarantee that vma is there.
>
>> In the second report T309 doesn't take any locks at all, since it
>> assumes that after checking atomic_dec_and_test(&mm->mm_users) the mm
>> has no other users, but then it does a write to vma.
>
> This one is tricky. I *assume* the mm cannot be generally accessible after
> mm_users drops to zero, but I'm not entirely sure about it.
> procfs? ptrace?
>
> The VMA is still accessible via rmap at this point. And I think it can be
> a problem:
>
> 		CPU0					CPU1
> exit_mmap()
>    // mmap_sem is *not* taken
>    munlock_vma_pages_all()
>      munlock_vma_pages_range()
>      						try_to_unmap_one()
> 						  down_read_trylock(&vma->vm_mm->mmap_sem))
> 						  !!(vma->vm_flags & VM_LOCKED) == true
>        vma->vm_flags &= ~VM_LOCKED;
>        <munlock the page>
>        						  mlock_vma_page(page);
> 						  // mlocked pages is leaked.
>
> The obvious solution is to take mmap_sem in exit path, but it would cause
> performance regression.
>
> Any comments?

Just so others don't repeat the paths that I already looked at:

- First I thought that try_to_unmap_one() has the page locked and 
munlock_vma_pages_range() will also lock it... but it doesn't.
- Then I thought that exit_mmap() will revisit the page anyway doing 
actual unmap. It would, if it's the one who has the page mapped, it will 
clear the mlock (see page_remove_rmap()). If it's not the last one, page 
will be left locked. So it won't be completely leaked, but still, it 
will be mlocked when it shouldn't.

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