Re: [PATCH v2] mm: slub: avoid deref of free pointer in sanity checks if object is invalid

[Vlastimil Babka <vbabka@suse.cz>]

On 7/25/25 08:49, Li Qiong wrote:
> For debugging, object_err() prints free pointer of the object.
> However, if check_valid_pointer() returns false for a object,
> dereferncing `object + s->offset` can lead to a crash. Therefore,
> print the object's address in such cases.
> 
> Fixes: bb192ed9aa71 ("mm/slub: Convert most struct page to struct slab by spatch")

That was the last commit to change the line, but the problem existed before,
AFAICS all the time, so I did:

    Fixes: 7656c72b5a63 ("SLUB: add macros for scanning objects in a slab")
    Cc: <stable@vger.kernel.org>

> Signed-off-by: Li Qiong <liqiong@nfschina.com>

Added to slab/for-next, thanks!

> ---
> v2:
> - rephrase the commit message, add comment for object_err().
> ---
>  mm/slub.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 31e11ef256f9..8b24f1cf3079 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1097,6 +1097,10 @@ static void print_trailer(struct kmem_cache *s, struct slab *slab, u8 *p)
>  			      size_from_object(s) - off);
>  }
>  
> +/*
> + * object - should be a valid object.
> + * check_valid_pointer(s, slab, object) should be true.
> + */
>  static void object_err(struct kmem_cache *s, struct slab *slab,
>  			u8 *object, const char *reason)
>  {
> @@ -1587,7 +1591,7 @@ static inline int alloc_consistency_checks(struct kmem_cache *s,
>  		return 0;
>  
>  	if (!check_valid_pointer(s, slab, object)) {
> -		object_err(s, slab, object, "Freelist Pointer check fails");
> +		slab_err(s, slab, "Invalid object pointer 0x%p", object);
>  		return 0;
>  	}
>