Re: Multiple potential races on vma->vm_flags

[Vlastimil Babka]

On 09/11/2015 05:29 PM, Vlastimil Babka wrote:
> 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.

More precisely, it does (in __munlock_pagevec()), but 
TestClearPageMlocked(page) doesn't happen under that lock.

> - 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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