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

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Message-ID: <431bf109-2bc7-4284-9b3a-7b36c329f25b@suse.cz>
Date: Mon, 28 Jul 2025 10:52:22 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Li Qiong <liqiong@...china.com>, Christoph Lameter <cl@...two.org>,
 David Rientjes <rientjes@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>
Cc: Roman Gushchin <roman.gushchin@...ux.dev>,
 Harry Yoo <harry.yoo@...cle.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] mm: slub: avoid deref of free pointer in sanity checks
 if object is invalid

On 7/25/25 18:47, Vlastimil Babka wrote:
> 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@...r.kernel.org>
> 
>> Signed-off-by: Li Qiong <liqiong@...china.com>
> 
> Added to slab/for-next, thanks!

Dropped based on Matthew's review

>> ---
>> 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;
>>  	}
>>  
>