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

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

On Thu, Aug 28, 2025 at 03:26:52AM +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.

Yikes, sorry my mistake.

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

You need a Cc: Stable.

> ---
>  mm/mremap.c | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
>
> diff --git a/mm/mremap.c b/mm/mremap.c
> index e618a706aff5..692acb0f9ea2 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -1771,18 +1771,17 @@ static unsigned long check_mremap_params(struct vma_remap_struct *vrm)
>  	 * for DOS-emu "duplicate shm area" thing. But
>  	 * a zero new-len is nonsensical.
>  	 */
> -	if (!vrm->new_len)
> -		return -EINVAL;

Please don't refactor code at the same time, especially as this is a hotfix.

This line is associated with the above comment.

> -
> -	/* Is the new length or address silly? */
> -	if (vrm->new_len > TASK_SIZE ||
> -	    vrm->new_addr > TASK_SIZE - vrm->new_len)
> +	if (!vrm->new_len || vrm->new_len > TASK_SIZE)
>  		return -EINVAL;

Yeah, it's cute but by removing the 'silly' comment you're making this
associated with the comment above and... yeah.

This should be:

	/*
	 * We allow a zero old-len as a special case
	 * for DOS-emu "duplicate shm area" thing. But
	 * a zero new-len is nonsensical.
	 */
	if (!vrm->new_len)
		return -EINVAL;

	/* 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;
> +

Obviously this bit is fine :)

Sorry again, this was just a mistake on my part.

>  	/* The new address must be page-aligned. */
>  	if (offset_in_page(vrm->new_addr))
>  		return -EINVAL;
> --
> 2.51.0.268.g9569e192d0-goog
>

I'm curious why only you guys have seen it, a theory is bionic is sending random
stuff to this parameter when unspecified, and glibc is not.

But obviously this fix is correct, and the original code needs fixing.

Please respin a v2 as per above.

Cheers, Lorenzo