Re: [PATCH v3] mm: slub: avoid deref of free pointer in sanity checks if object is invalid

[liqiong]

在 2025/7/29 21:41, Harry Yoo 写道:
> On Tue, Jul 29, 2025 at 04:14:55PM +0800, 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.
> As the code changed a bit, I think the commit message could better reflect
> what this patch actually does.
Yes.

>
>> Fixes: bb192ed9aa71 ("mm/slub: Convert most struct page to struct slab by spatch")
> As Vlastimil mentioned in previous version, this is not the first commit
> that introduced this problem.
>
>> Cc: <stable@xxxxxxxxxxxxxxx>
>> Signed-off-by: Li Qiong <liqiong@xxxxxxxxxxxx>
>> ---
>> v2:
>> - rephrase the commit message, add comment for object_err().
>> v3:
>> - check object pointer in object_err().
>> ---
>>  mm/slub.c | 8 ++++++--
>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 31e11ef256f9..d3abae5a2193 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -1104,7 +1104,11 @@ static void object_err(struct kmem_cache *s, struct slab *slab,
>>  		return;
>>  
>>  	slab_bug(s, reason);
>> -	print_trailer(s, slab, object);
>> +	if (!check_valid_pointer(s, slab, object)) {
>> +		print_slab_info(slab);
>> +		pr_err("invalid object 0x%p\n", object);
> Can we just handle this inside print_trailer() because that's the function
> that prints the object's free pointer, metadata, etc.?
Maybe it's clearer ,  if  object pointer being invalid, don't enter print_trailer()，
print_trailer() prints  valid object.

>
> Also, the message should start with a capital letter.
>
> and "invalid object" sounds misleading because it's the pointer
> that is invalid. Perhaps simply "Invalid pointer 0x%p\n"?
> (What would be the most comprehensive message here? :P)

Make sense,   will change it.
>
>> +	} else
>> +		print_trailer(s, slab, object);
>>  	add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
>>  
>>  	WARN_ON(1);
>> @@ -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, "Freelist Pointer(0x%p) check fails", object);
>>  		return 0;
> Do we really need this hunk after making object_err() resiliant
> against wild pointers?

That's the origin issue,   it may be  inappropriate to use object_err(), if check_valid_pointer being false.

>
>>  	}