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

[Li Qiong]

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")
Signed-off-by: Li Qiong <liqiong@xxxxxxxxxxxx>
---
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;
 	}
 
-- 
2.30.2