[PATCH] slab: fix clearing freelist in free_deferred_objects()

[Vlastimil Babka]

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-by: kernel test robot <oliver.sang@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-lkp/202510101652.7921fdc6-lkp@xxxxxxxxx
Acked-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
will be added to slab/for-next-fixes and sent later this week
---
 mm/slub.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index b1f15598fbfd..64c17afc375b 100644
--- 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_);
 	}
 

---
base-commit: 3a8660878839faadb4f1a6dd72c3179c1df56787
change-id: 20251014-fix-freelist-29915edcc2f4

Best regards,
-- 
Vlastimil Babka <vbabka@xxxxxxx>