mm/slub: reset KASAN tag in defer_free() before accessing freed memory

When CONFIG_SLUB_TINY is enabled, kfree_nolock() calls kasan_slab_free()
before defer_free(). On ARM64 with MTE (Memory Tagging Extension),
kasan_slab_free() poisons the memory and changes the tag from the
original (e.g., 0xf3) to a poison tag (0xfe).

When defer_free() then tries to write to the freed object to build the
deferred free list via llist_add(), the pointer still has the old tag,
causing a tag mismatch and triggering a KASAN use-after-free report:

  BUG: KASAN: slab-use-after-free in defer_free+0x3c/0xbc mm/slub.c:6537
  Write at addr f3f000000854f020 by task kworker/u8:6/983
  Pointer tag: [f3], memory tag: [fe]

Fix this by calling kasan_reset_tag() before accessing the freed memory.
This is safe because defer_free() is part of the allocator itself and is
expected to manipulate freed memory for bookkeeping purposes.

Fixes: af92793e52 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
Cc: stable@vger.kernel.org
Reported-by: syzbot+7a25305a76d872abcfa1@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=7a25305a76d872abcfa1
Tested-by: syzbot+7a25305a76d872abcfa1@syzkaller.appspotmail.com
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20251210022024.3255826-1-kartikey406@gmail.com
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

mm/slub.c

@@ -6539,6 +6539,8 @@ static void defer_free(struct kmem_cache *s, void *head)
 
 	guard(preempt)();
 
+	head = kasan_reset_tag(head);
+
 	df = this_cpu_ptr(&defer_free_objects);
 	if (llist_add(head + s->offset, &df->objects))
 		irq_work_queue(&df->work);