Re: [PATCH] mm/mmap_lock: Reset maple state on lock_vma_under_rcu() retry

[Vlastimil Babka]

On 11/11/25 22:56, Liam R. Howlett wrote:
> The retry in lock_vma_under_rcu() drops the rcu read lock before
> reacquiring the lock and trying again.  This may cause a use-after-free
> if the maple node the maple state was using was freed.
> 
> The maple state is protected by the rcu read lock.  When the lock is
> dropped, the state cannot be reused as it tracks pointers to objects
> that may be freed during the time where the lock was not held.
> 
> Any time the rcu read lock is dropped, the maple state must be
> invalidated.  Resetting the address and state to MA_START is the safest
> course of action, which will result in the next operation starting from
> the top of the tree.
> 
> Prior to commit 0b16f8bed19c ("mm: change vma_start_read() to drop RCU
> lock on failure"), the rcu read lock was dropped and NULL was returned,
> so the retry would not have happened.  However, now that the read lock
> is dropped regardless of the return, we may use a freed maple tree node
> cached in the maple state on retry.
> 
> Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 0b16f8bed19c ("mm: change vma_start_read() to drop RCU lock on failure")

The commit is 6.18-rc1 so we don't need Cc: stable, but it's a mm-hotfixes
material that must go to Linus before 6.18.

> Reported-by: syzbot+131f9eb2b5807573275c@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=131f9eb2b5807573275c
> Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  mm/mmap_lock.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
> index 39f341caf32c0..f2532af6208c0 100644
> --- a/mm/mmap_lock.c
> +++ b/mm/mmap_lock.c
> @@ -257,6 +257,7 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
>  		if (PTR_ERR(vma) == -EAGAIN) {
>  			count_vm_vma_lock_event(VMA_LOCK_MISS);
>  			/* The area was replaced with another one */
> +			mas_set(&mas, address);
>  			goto retry;
>  		}
>