Re: Multiple potential races on vma->vm_flags

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Wed, Sep 09, 2015 at 05:27:16PM +0200, Vlastimil Babka wrote:
> On 09/07/2015 01:40 PM, Kirill A. Shutemov wrote:
> >On Sun, Sep 06, 2015 at 03:21:05PM -0400, Sasha Levin wrote:
> >>==================================================================
> >>ThreadSanitizer: data-race in munlock_vma_pages_range
> >>
> >>Write of size 8 by thread T378 (K2633, CPU3):
> >>  [<ffffffff81212579>] munlock_vma_pages_range+0x59/0x3e0 mm/mlock.c:425
> >>  [<ffffffff81212ac9>] mlock_fixup+0x1c9/0x280 mm/mlock.c:549
> >>  [<ffffffff81212ccc>] do_mlock+0x14c/0x180 mm/mlock.c:589
> >>  [<     inlined    >] SyS_munlock+0x74/0xb0 SYSC_munlock mm/mlock.c:651
> >>  [<ffffffff812130b4>] SyS_munlock+0x74/0xb0 mm/mlock.c:643
> >>  [<ffffffff81eb352e>] entry_SYSCALL_64_fastpath+0x12/0x71
> >>arch/x86/entry/entry_64.S:186
> >
> >...
> >
> >>Previous read of size 8 by thread T398 (K2623, CPU2):
> >>  [<ffffffff8121d198>] try_to_unmap_one+0x78/0x4f0 mm/rmap.c:1208
> >>  [<     inlined    >] rmap_walk+0x147/0x450 rmap_walk_file mm/rmap.c:1540
> >>  [<ffffffff8121e7b7>] rmap_walk+0x147/0x450 mm/rmap.c:1559
> >>  [<ffffffff8121ef72>] try_to_munlock+0xa2/0xc0 mm/rmap.c:1423
> >>  [<ffffffff81211bb0>] __munlock_isolated_page+0x30/0x60 mm/mlock.c:129
> >>  [<ffffffff81212066>] __munlock_pagevec+0x236/0x3f0 mm/mlock.c:331
> >>  [<ffffffff812128a0>] munlock_vma_pages_range+0x380/0x3e0 mm/mlock.c:476
> >>  [<ffffffff81212ac9>] mlock_fixup+0x1c9/0x280 mm/mlock.c:549
> >>  [<ffffffff81212ccc>] do_mlock+0x14c/0x180 mm/mlock.c:589
> >>  [<     inlined    >] SyS_munlock+0x74/0xb0 SYSC_munlock mm/mlock.c:651
> >>  [<ffffffff812130b4>] SyS_munlock+0x74/0xb0 mm/mlock.c:643
> >>  [<ffffffff81eb352e>] entry_SYSCALL_64_fastpath+0x12/0x71
> >>arch/x86/entry/entry_64.S:186
> >
> >Okay, the detected race is mlock/munlock vs. rmap.
> >
> >On rmap side we check vma->vm_flags in few places without taking
> >vma->vm_mm->mmap_sem. The vma cannot be freed since we hold i_mmap_rwsem
> >or anon_vma_lock, but nothing prevent vma->vm_flags from changing under
> >us.
> >
> >In this particular case, speculative check in beginning of
> >try_to_unmap_one() is fine, since we re-check it under mmap_sem later in
> >the function.
> >
> >False-negative is fine too here, since we will mlock the page in
> >__mm_populate() on mlock side after mlock_fixup().
> >
> >BUT.
> >
> >We *must* have all speculative vm_flags accesses wrapped READ_ONCE() to
> >avoid all compiler trickery, like duplication vm_flags access with
> >inconsistent results.
> 
> Doesn't taking a semaphore, as in try_to_unmap_one(), already imply a
> compiler barrier forcing vm_flags to be re-read?

Yes, but it doesn't prevent compiler from generation multiple reads from
vma->vm_flags and it may blow up if two values doesn't match.

> >I looked only on VM_LOCKED checks, but there are few other flags checked
> >in rmap. All of them must be handled carefully. At least READ_ONCE() is
> >required.
> >
> >Other solution would be to introduce per-vma spinlock to protect
> >vma->vm_flags and probably other vma fields and offload this duty
> >from mmap_sem.
> >But that's much bigger project.
> 
> Sounds like an overkill, unless we find something more serious than this.

May be...

-- 
 Kirill A. Shutemov

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