On Tue, Jun 20, 2023 at 11:57 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> Current implementation of vma_start_read() checks VMA for being locked
> before taking vma->vm_lock and then checks that again. This mechanism
> fails to detect a case when the VMA gets write-locked, modified and
> unlocked after the first check but before the vma->vm_lock is obtained.
> While this is not strictly a problem (vma_start_read would not produce
> a false unlocked result), this allows it to successfully lock a VMA which
> got detached from the VMA tree while vma_start_read was locking it.
> New condition checks for any change in vma->vm_lock_seq after we obtain
> vma->vm_lock and will cause vma_start_read() to fail if the above race
> occurs.
Just a friendly ping for feedback.
I know most people were busy fixing the stack expansion problem and
these patches are not urgent, so no rush. If nobody reviews them, I'll
ping again next week.
>
> Fixes: 5e31275cc997 ("mm: add per-VMA lock and helper functions to control it")
> Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> ---
> include/linux/mm.h | 19 ++++++++++---------
> 1 file changed, 10 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 27ce77080c79..8410da79c570 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -639,23 +639,24 @@ static inline void vma_numab_state_free(struct vm_area_struct *vma) {}
> */
> static inline bool vma_start_read(struct vm_area_struct *vma)
> {
> - /* Check before locking. A race might cause false locked result. */
> - if (vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))
> + int vm_lock_seq = READ_ONCE(vma->vm_lock_seq);
> +
> + /*
> + * Check if VMA is locked before taking vma->vm_lock. A race or
> + * mm_lock_seq overflow might cause false locked result.
> + */
> + if (vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))
> return false;
>
> if (unlikely(down_read_trylock(&vma->vm_lock->lock) == 0))
> return false;
>
> - /*
> - * Overflow might produce false locked result.
> - * False unlocked result is impossible because we modify and check
> - * vma->vm_lock_seq under vma->vm_lock protection and mm->mm_lock_seq
> - * modification invalidates all existing locks.
> - */
> - if (unlikely(vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))) {
> + /* Fail if VMA was write-locked after we checked it earlier */
> + if (unlikely(vm_lock_seq != READ_ONCE(vma->vm_lock_seq))) {
> up_read(&vma->vm_lock->lock);
> return false;
> }
> +
> return true;
> }
>
> --
> 2.41.0.162.gfafddb0af9-goog
>
