linux-kernel - Re: [PATCH] mm/mremap: fix regression in vrm->new

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <8a4dc910-5237-48aa-8abb-a6d5044bc290@lucifer.local>
Date: Thu, 28 Aug 2025 06:38:09 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Carlos Llamas <cmllamas@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Pedro Falcato <pfalcato@...e.de>, kernel-team@...roid.com,
        linux-kernel@...r.kernel.org,
        "open list:MEMORY MAPPING" <linux-mm@...ck.org>
Subject: Re: [PATCH] mm/mremap: fix regression in vrm->new_addr check

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@...gle.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