Re: [PATCH v6] mm/slub: avoid accessing metadata when pointer is invalid in object_err()

[Vlastimil Babka <vbabka@suse.cz>]

On 8/4/25 04:57, Li Qiong wrote:
> object_err() reports details of an object for further debugging, such as
> the freelist pointer, redzone, etc. However, if the pointer is invalid,
> attempting to access object metadata can lead to a crash since it does
> not point to a valid object.
> 
> In case the pointer is NULL or check_valid_pointer() returns false for
> the pointer, only print the pointer value and skip accessing metadata.

We should explain that this is not theoretical so justify the stable cc, so
I would add:

One known path to the crash is when alloc_consistency_checks() determines
the pointer to the allocated object is invalid beause of a freelist
corruption, and calls object_err() to report it. The debug code should
report and handle the corruption gracefully and not crash in the process.

If you agree, I can do this when picking up the patch after merge window, no
need to resend.
> Fixes: 81819f0fc828 ("SLUB core")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Li Qiong <liqiong@nfschina.com>
> ---
> v2:
> - rephrase the commit message, add comment for object_err().
> v3:
> - check object pointer in object_err().
> v4:
> - restore changes in alloc_consistency_checks().
> v5:
> - rephrase message, fix code style.
> v6:
> - add checking 'object' if NULL.
> ---
>  mm/slub.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 31e11ef256f9..972cf2bb2ee6 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1104,7 +1104,12 @@ static void object_err(struct kmem_cache *s, struct slab *slab,
>  		return;
>  
>  	slab_bug(s, reason);
> -	print_trailer(s, slab, object);
> +	if (!object || !check_valid_pointer(s, slab, object)) {
> +		print_slab_info(slab);
> +		pr_err("Invalid pointer 0x%p\n", object);
> +	} else {
> +		print_trailer(s, slab, object);
> +	}
>  	add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
>  
>  	WARN_ON(1);