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

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: 7a8e71bc619d ("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
Link: https://lore.kernel.org/all/ddff7c7d-c0c3-4780-808f-9a83268bbf0c@linux.ibm.com
Link: https://lore.kernel.org/linux-mm/aZu9G9mVIVzSm6Ft@hyeyoo
Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>