mm/slab: initialize slab->stride early to avoid memory ordering issues

When alloc_slab_obj_exts() is called later (instead of during slab
allocation and initialization), slab->stride and slab->obj_exts are
updated after the slab is already accessible by multiple CPUs.

The current implementation does not enforce memory ordering between
slab->stride and slab->obj_exts. For correctness, slab->stride must be
visible before slab->obj_exts. Otherwise, concurrent readers may observe
slab->obj_exts as non-zero while stride is still stale.

With stale slab->stride, slab_obj_ext() could return the wrong obj_ext.
This could cause two problems:

  - obj_cgroup_put() is called on the wrong objcg, leading to
    a use-after-free due to incorrect reference counting [1] by
    decrementing the reference count more than it was incremented.

  - refill_obj_stock() is called on the wrong objcg, leading to
    a page_counter overflow [2] by uncharging more memory than charged.

Fix this by unconditionally initializing slab->stride in
alloc_slab_obj_exts_early(), before the need_slab_obj_exts() check.
In the case of SLAB_OBJ_EXT_IN_OBJ, it is overridden in the function.

This ensures updates to slab->stride become visible before the slab
can be accessed by other CPUs via the per-node partial slab list
(protected by spinlock with acquire/release semantics).

Thanks to Shakeel Butt for pointing out this issue [3].

[vbabka@kernel.org: the bug reports [1] and [2] are not yet fully fixed,
 with investigation ongoing, but it is nevertheless a step in the right
 direction to only set stride once after allocating the slab and not
 change it later ]

Fixes: 7a8e71b ("mm/slab: use stride to access slabobj_ext")
Reported-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Link: https://lore.kernel.org/lkml/ca241daa-e7e7-4604-a48d-de91ec9184a5@linux.ibm.com [1]
Link: https://lore.kernel.org/all/ddff7c7d-c0c3-4780-808f-9a83268bbf0c@linux.ibm.com [2]
Link: https://lore.kernel.org/linux-mm/aZu9G9mVIVzSm6Ft@hyeyoo [3]
Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

Author: hygoni

mm/slub.c

@@ -2196,7 +2196,6 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
 retry:
 	old_exts = READ_ONCE(slab->obj_exts);
 	handle_failed_objexts_alloc(old_exts, vec, objects);
-	slab_set_stride(slab, sizeof(struct slabobj_ext));
 
 	if (new_slab) {
 		/*
@@ -2272,6 +2271,9 @@ static void alloc_slab_obj_exts_early(struct kmem_cache *s, struct slab *slab)
 	void *addr;
 	unsigned long obj_exts;
 
+	/* Initialize stride early to avoid memory ordering issues */
+	slab_set_stride(slab, sizeof(struct slabobj_ext));
+
 	if (!need_slab_obj_exts(s))
 		return;
 
@@ -2288,7 +2290,6 @@ static void alloc_slab_obj_exts_early(struct kmem_cache *s, struct slab *slab)
 		obj_exts |= MEMCG_DATA_OBJEXTS;
 #endif
 		slab->obj_exts = obj_exts;
-		slab_set_stride(slab, sizeof(struct slabobj_ext));
 	} else if (s->flags & SLAB_OBJ_EXT_IN_OBJ) {
 		unsigned int offset = obj_exts_offset_in_object(s);