Re: [PATCH v2] mm/mremap: fix regression in vrm->new_addr check

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

(For future reference) please send separately rather than in reply to first
:)

Otherwise harder for me to find your series!

On Thu, Aug 28, 2025 at 02:26:56PM +0000, Carlos Llamas wrote:
> Commit 3215eaceca87 ("mm/mremap: refactor initial parameter sanity
> checks") moved the sanity check for vrm->new_addr from mremap_to() to
> check_mremap_params().
>
> However, this caused a regression as vrm->new_addr is now checked even
> when MREMAP_FIXED and MREMAP_DONTUNMAP flags are not specified. In this
> case, vrm->new_addr can be garbage and create unexpected failures.
>
> Fix this by moving the new_addr check after the vrm_implies_new_addr()
> guard. This ensures that the new_addr is only checked when the user has
> specified one explicitly.
>
> Cc: stable@vger.kernel.org

Yeah oopsies on me suggesting this :P losing track of my own patches :)

> Fixes: 3215eaceca87 ("mm/mremap: refactor initial parameter sanity checks")
> Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
> Signed-off-by: Carlos Llamas <cmllamas@google.com>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

And again thanks so much for this! :)

> ---
> v2:
>  - split out vrm->new_len into individual checks
>  - cc stable, collect tags
>
> v1:
> https://lore.kernel.org/all/20250828032653.521314-1-cmllamas@google.com/
>
>  mm/mremap.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/mm/mremap.c b/mm/mremap.c
> index e618a706aff5..35de0a7b910e 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -1774,15 +1774,18 @@ static unsigned long check_mremap_params(struct vma_remap_struct *vrm)
>  	if (!vrm->new_len)
>  		return -EINVAL;
>
> -	/* Is the new length or address silly? */
> -	if (vrm->new_len > TASK_SIZE ||
> -	    vrm->new_addr > TASK_SIZE - vrm->new_len)
> +	/* Is the new length silly? */
> +	if (vrm->new_len > TASK_SIZE)
>  		return -EINVAL;
>
>  	/* Remainder of checks are for cases with specific new_addr. */
>  	if (!vrm_implies_new_addr(vrm))
>  		return 0;
>
> +	/* Is the new address silly? */
> +	if (vrm->new_addr > TASK_SIZE - vrm->new_len)
> +		return -EINVAL;
> +
>  	/* The new address must be page-aligned. */
>  	if (offset_in_page(vrm->new_addr))
>  		return -EINVAL;
> --
> 2.51.0.268.g9569e192d0-goog
>

Cheers, Lorenzo