Re: [PATCH v2] vma: Detect infinite loop in vma tree

[Lorenzo Stoakes]

On Thu, Oct 31, 2024 at 03:36:08PM -0400, Liam R. Howlett wrote:
> From: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
>
> There have been no reported infinite loops in the tree, but checking the
> detection of an infinite loop during validation is simple enough.  Add
> the detection to the validate_mm() function so that error reports are
> clear and don't just report stalls.
>
> This does not protect against internal maple tree issues, but it does
> detect too many vmas being returned from the tree.
>
> The variance of +10 is to allow for the debugging output to be more useful for
> nearly correct counts.  In the event of more than 10 over the map_count, the
> count will be set to -1 for easier identification of a potential infinite loop.
>
> Note that the mmap lock is held to ensure a consistent tree state during the
> validation process.
>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>

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

> Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Jann Horn <jannh@xxxxxxxxxx>
> ---
>  mm/vma.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/mm/vma.c b/mm/vma.c
> index 68138e8c153e..283e6bc4884f 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -615,7 +615,10 @@ void validate_mm(struct mm_struct *mm)
>  			anon_vma_unlock_read(anon_vma);
>  		}
>  #endif
> -		i++;
> +		if (++i > mm->map_count + 10) {
> +			i = -1;
> +			break;
> +		}
>  	}
>  	if (i != mm->map_count) {
>  		pr_emerg("map_count %d vma iterator %d\n", mm->map_count, i);
> --
> 2.43.0
>