Re: [PATCH v5] mm/userfaultfd: detect VMA type change after copy retry in mfill_copy_folio_retry()

[Mike Rapoport]

On Thu, Apr 09, 2026 at 01:06:53PM +0100, David Carlier wrote:
> mfill_copy_folio_retry() drops mmap_lock for the copy_from_user() call.
> During this window, the VMA can be replaced with a different type (e.g.
> hugetlb), making the caller's ops pointer stale. Subsequent use of the
> stale ops can lead to incorrect folio handling or a kernel crash.
> 
> Pass the caller's ops into mfill_copy_folio_retry() and compare against
> the current vma_uffd_ops() after re-acquiring the lock. Return -EAGAIN
> if they differ so the operation can be retried.
> 
> Fixes: 59da5c32ffa3 ("userfaultfd: mfill_atomic(): remove retry logic")
> Signed-off-by: David Carlier <devnexen@xxxxxxxxx>

Acked-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>

> ---
>  mm/userfaultfd.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 481ec7eb4442..214923a411c1 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -443,7 +443,9 @@ static int mfill_copy_folio_locked(struct folio *folio, unsigned long src_addr)
>  	return ret;
>  }
>  
> -static int mfill_copy_folio_retry(struct mfill_state *state, struct folio *folio)
> +static int mfill_copy_folio_retry(struct mfill_state *state,
> +				  const struct vm_uffd_ops *ops,
> +				  struct folio *folio)
>  {
>  	unsigned long src_addr = state->src_addr;
>  	void *kaddr;
> @@ -465,6 +467,14 @@ static int mfill_copy_folio_retry(struct mfill_state *state, struct folio *folio
>  	if (err)
>  		return err;
>  
> +	/*
> +	 * The VMA type may have changed while the lock was dropped
> +	 * (e.g. replaced with a hugetlb mapping), making the caller's
> +	 * ops pointer stale.
> +	 */
> +	if (vma_uffd_ops(state->vma) != ops)
> +		return -EAGAIN;
> +
>  	err = mfill_establish_pmd(state);
>  	if (err)
>  		return err;
> @@ -495,7 +505,7 @@ static int __mfill_atomic_pte(struct mfill_state *state,
>  		 * will take care of unlocking if needed.
>  		 */
>  		if (unlikely(ret)) {
> -			ret = mfill_copy_folio_retry(state, folio);
> +			ret = mfill_copy_folio_retry(state, ops, folio);
>  			if (ret)
>  				goto err_folio_put;
>  		}
> -- 
> 2.53.0
> 

-- 
Sincerely yours,
Mike.