Re: [PATCH] mm/mremap: reset unfaulted VMA page offset for MREMAP_DONTUNMAP

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 8/25/26 9:55 AM, Lorenzo Stoakes (ARM) wrote:
> Uniquely an mremap() invocation using the MREMAP_DONTUNMAP flag can reset
> a faulted VMA into an unfaulted one.
> 
> It does so after the page tables have been moved to the copied VMA with
> MREMAP_DONTUNMAP leaving the old VMA in place which is naturally unfaulted
> as the page tables it had are no longer present.
> 
> However, in doing so, it violates the invariant that the anonymous page
> offset of an unfaulted VMA is vma->vm_start >> PAGE_SHIFT.
> 
> This is because a VMA may have been faulted in, mremap()'d (causing a delta
> between its page offset and vma->vm_start >> PAGE_SHIFT), and then
> mremap()'d again with MREMAP_DONTUNMAP resulting in the unfaulting.
> 
> This condition is a violation of a fundamental assumption in mm, but now
> also triggers an assert in assert_sane_pgoff() which explicitly checks for
> this condition.

Oof. So what's the worst thing that could happen before the assert was
added? We'd use the "unfaulted" state to allow a merge, but the wrong
pgoff could mess up the result of the merge somehow?

> Correct it by resetting the VMA's page offset at the point of completing
> the MREMAP_DONTUNMAP operation.
> 
> Reported-by: syzbot+f12658786a4153df5113@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/6a87853b.ae6ddae5.3da009.0023.GAE@google.com/
> Fixes: 1583aa278f5f ("mm: mremap: unlink anon_vmas when mremap with MREMAP_DONTUNMAP success")
> Cc: stable@vger.kernel.org
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

> ---
>  mm/mremap.c | 22 +++++++++++++++++-----
>  1 file changed, 17 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/mremap.c b/mm/mremap.c
> index e8df5cdb0ac9..2b4b523a86b8 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -1331,18 +1331,30 @@ static void dontunmap_complete(struct vma_remap_struct *vrm,
>  {
>  	unsigned long start = vrm->addr;
>  	unsigned long end = vrm->addr + vrm->old_len;
> -	unsigned long old_start = vrm->vma->vm_start;
> -	unsigned long old_end = vrm->vma->vm_end;
> +	struct vm_area_struct *vma = vrm->vma;
> +	unsigned long old_start = vma->vm_start;
> +	unsigned long old_end = vma->vm_end;
>  
>  	/* We always clear VMA_LOCKED[ONFAULT]_BIT on the old VMA. */
> -	vma_clear_flags_mask(vrm->vma, VMA_LOCKED_MASK);
> +	vma_clear_flags_mask(vma, VMA_LOCKED_MASK);
>  
>  	/*
>  	 * anon_vma links of the old vma is no longer needed after its page
>  	 * table has been moved.
>  	 */
> -	if (new_vma != vrm->vma && start == old_start && end == old_end)
> -		unlink_anon_vmas(vrm->vma);
> +	if (new_vma != vma && start == old_start && end == old_end) {
> +		const pgoff_t pgoff_unfaulted = vma->vm_start >> PAGE_SHIFT;
> +
> +		unlink_anon_vmas(vma);
> +		/*
> +		 * The VMA is now unfaulted and it is an invariant that
> +		 * unfaulted anonymous VMAs have page offset equal to
> +		 * vma->vm_start >> PAGE_SHIFT.
> +		 */
> +		vma_set_anon_pgoff(vma, pgoff_unfaulted);
> +		if (vma_is_anonymous(vma) && !vma->vm_file)
> +			vma_set_pgoff(vma, pgoff_unfaulted);
> +	}
>  
>  	/* Because we won't unmap we don't need to touch locked_vm. */
>  }
> 
> ---
> base-commit: efecab401cb15fd3bb9bc05990609acb6b267ff2
> change-id: 20260824-fix-mremap-dontunmap-pgoff-a687134e995e
> 
> Best regards,