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

[Harry Yoo]

On Wed, Jul 30, 2025 at 09:46:09AM +0800, liqiong wrote:
> 在 2025/7/29 21:41, Harry Yoo 写道:
> > On Tue, Jul 29, 2025 at 04:14:55PM +0800, Li Qiong wrote:
> >> 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.

Please don't forget to update Fixes: tag :)

> >> 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.

You're probably right. No strong opinion.
object_err() is the only user anyway.

> >> +	} 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.

That was the original issue, but you're making it not crash even if
with bad pointers are passed?

> >>  	}
> 

-- 
Cheers,
Harry / Hyeonggon