slab: Avoid race on slab->obj_exts in alloc_slab_obj_exts

If two competing threads enter alloc_slab_obj_exts() and one of them
fails to allocate the object extension vector, it might override the
valid slab->obj_exts allocated by the other thread with
OBJEXTS_ALLOC_FAIL. This will cause the thread that lost this race and
expects a valid pointer to dereference a NULL pointer later on.

Update slab->obj_exts atomically using cmpxchg() to avoid
slab->obj_exts overrides by racing threads.

Thanks for Vlastimil and Suren's help with debugging.

Fixes: f7381b9 ("slab: mark slab->obj_exts allocation failures unconditionally")
Cc: <[email protected]>
Suggested-by: Suren Baghdasaryan <[email protected]>
Signed-off-by: Hao Ge <[email protected]>
Reviewed-by: Harry Yoo <[email protected]>
Reviewed-by: Suren Baghdasaryan <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Vlastimil Babka <[email protected]>

Author: GeHao01994

mm/slub.c

@@ -2054,7 +2054,7 @@ static inline void mark_objexts_empty(struct slabobj_ext *obj_exts)
 
 static inline void mark_failed_objexts_alloc(struct slab *slab)
 {
-	slab->obj_exts = OBJEXTS_ALLOC_FAIL;
+	cmpxchg(&slab->obj_exts, 0, OBJEXTS_ALLOC_FAIL);
 }
 
 static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
@@ -2136,6 +2136,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
 #ifdef CONFIG_MEMCG
 	new_exts |= MEMCG_DATA_OBJEXTS;
 #endif
+retry:
 	old_exts = READ_ONCE(slab->obj_exts);
 	handle_failed_objexts_alloc(old_exts, vec, objects);
 	if (new_slab) {
@@ -2145,8 +2146,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
 		 * be simply assigned.
 		 */
 		slab->obj_exts = new_exts;
-	} else if ((old_exts & ~OBJEXTS_FLAGS_MASK) ||
-		   cmpxchg(&slab->obj_exts, old_exts, new_exts) != old_exts) {
+	} else if (old_exts & ~OBJEXTS_FLAGS_MASK) {
 		/*
 		 * If the slab is already in use, somebody can allocate and
 		 * assign slabobj_exts in parallel. In this case the existing
@@ -2158,6 +2158,9 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
 		else
 			kfree(vec);
 		return 0;
+	} else if (cmpxchg(&slab->obj_exts, old_exts, new_exts) != old_exts) {
+		/* Retry if a racing thread changed slab->obj_exts from under us. */
+		goto retry;
 	}
 
 	if (allow_spin)