slab: fix clearing freelist in free_deferred_objects()

defer_free() links pending objects using the slab's freelist offset
which is fine as they are not free yet. free_deferred_objects() then
clears this pointer to avoid confusing the debugging consistency checks
that may be enabled for the cache.

However, with CONFIG_SLAB_FREELIST_HARDENED, even the NULL pointer needs
to be encoded appropriately using set_freepointer(), otherwise it's
decoded as something else and triggers the consistency checks, as found
by the kernel test robot.

Use set_freepointer() to prevent the issue.

Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
Reported-and-tested-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202510101652.7921fdc6-lkp@intel.com
Acked-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

diff --git a/mm/slub.c b/mm/slub.c
index b1f15598fbfd2320df7d7f7a306c6a2c311d378e..13ae4491136ac6de78402bb3e08bedf012bb89f8 100644 (file)
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -6443,15 +6443,16 @@ static void free_deferred_objects(struct irq_work *work)
                slab = virt_to_slab(x);
                s = slab->slab_cache;
 
+               /* Point 'x' back to the beginning of allocated object */
+               x -= s->offset;
+
                /*
                 * We used freepointer in 'x' to link 'x' into df->objects.
                 * Clear it to NULL to avoid false positive detection
                 * of "Freepointer corruption".
                 */
-               *(void **)x = NULL;
+               set_freepointer(s, x, NULL);
 
-               /* Point 'x' back to the beginning of allocated object */
-               x -= s->offset;
                __slab_free(s, slab, x, x, 1, _THIS_IP_);
        }