[PATCH] slub: Fixes freepointer encoding for single free

[Nicolas Bouchinet <nicolas.bouchinet@clip-os.org>]

From: Nicolas Bouchinet <nicolas.bouchinet@ssi.gouv.fr>

Commit 284f17ac13fe ("mm/slub: handle bulk and single object freeing
separately") splits single and bulk object freeing in two functions
slab_free() and slab_free_bulk() which leads slab_free() to call
slab_free_hook() directly instead of slab_free_freelist_hook().

If `init_on_free` is set, slab_free_hook() zeroes the object.
Afterward, if `slub_debug=F` and `CONFIG_SLAB_FREELIST_HARDENED` are
set, the do_slab_free() slowpath executes freelist consistency
checks and try to decode a zeroed freepointer which leads to a
"Freepointer corrupt" detection in check_object().

Object's freepointer thus needs to be properly set using
set_freepointer() after init_on_free.

To reproduce, set `slub_debug=FU init_on_free=1 log_level=7` on the
command line of a kernel build with `CONFIG_SLAB_FREELIST_HARDENED=y`.

dmesg sample log:
[   10.708715] =============================================================================
[   10.710323] BUG kmalloc-rnd-05-32 (Tainted: G    B           T ): Freepointer corrupt
[   10.712695] -----------------------------------------------------------------------------
[   10.712695]
[   10.712695] Slab 0xffffd8bdc400d580 objects=32 used=4 fp=0xffff9d9a80356f80 flags=0x200000000000a00(workingset|slab|node=0|zone=2)
[   10.716698] Object 0xffff9d9a80356600 @offset=1536 fp=0x7ee4f480ce0ecd7c
[   10.716698]
[   10.716698] Bytes b4 ffff9d9a803565f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
[   10.720703] Object   ffff9d9a80356600: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
[   10.720703] Object   ffff9d9a80356610: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
[   10.724696] Padding  ffff9d9a8035666c: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
[   10.724696] Padding  ffff9d9a8035667c: 00 00 00 00                                      ....
[   10.724696] FIX kmalloc-rnd-05-32: Object at 0xffff9d9a80356600 not freed

Signed-off-by: Nicolas Bouchinet <nicolas.bouchinet@ssi.gouv.fr>
---
 mm/slub.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index 3aa12b9b323d9..71dbff9ad8f17 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -4342,10 +4342,16 @@ static __fastpath_inline
 void slab_free(struct kmem_cache *s, struct slab *slab, void *object,
 	       unsigned long addr)
 {
+	bool init = false;
+
 	memcg_slab_free_hook(s, slab, &object, 1);
+	init = slab_want_init_on_free(s);
 
-	if (likely(slab_free_hook(s, object, slab_want_init_on_free(s))))
+	if (likely(slab_free_hook(s, object, init))) {
+		if (init)
+			set_freepointer(s, object, NULL);
 		do_slab_free(s, slab, object, object, 1, addr);
+	}
 }
 
 static __fastpath_inline
-- 
2.44.0