[PATCH] mm: slub: Fix slab walking for init_on_free

[Laura Abbott <labbott@redhat.com>]

To properly clear the slab on free with slab_want_init_on_free,
we walk the list of free objects using get_freepointer/set_freepointer.
The value we get from get_freepointer may not be valid. This
isn't an issue since an actual value will get written later
but this means there's a chance of triggering a bug if we use
this value with set_freepointer:

[    4.478342] kernel BUG at mm/slub.c:306!
[    4.482437] invalid opcode: 0000 [#1] PREEMPT PTI
[    4.485750] CPU: 0 PID: 0 Comm: swapper Not tainted 5.2.0-05754-g6471384a #4
[    4.490635] RIP: 0010:kfree+0x58a/0x5c0
[    4.493679] Code: 48 83 05 78 37 51 02 01 0f 0b 48 83 05 7e 37 51 02 01 48 83 05 7e 37 51 02 01 48 83 05 7e 37 51 02 01 48 83 05 d6 37 51 02 01 <0f> 0b 48 83 05 d4 37 51 02 01 48 83 05 d4 37 51 02 01 48 83 05 d4
[    4.506827] RSP: 0000:ffffffff82603d90 EFLAGS: 00010002
[    4.510475] RAX: ffff8c3976c04320 RBX: ffff8c3976c04300 RCX: 0000000000000000
[    4.515420] RDX: ffff8c3976c04300 RSI: 0000000000000000 RDI: ffff8c3976c04320
[    4.520331] RBP: ffffffff82603db8 R08: 0000000000000000 R09: 0000000000000000
[    4.525288] R10: ffff8c3976c04320 R11: ffffffff8289e1e0 R12: ffffd52cc8db0100
[    4.530180] R13: ffff8c3976c01a00 R14: ffffffff810f10d4 R15: ffff8c3976c04300
[    4.535079] FS:  0000000000000000(0000) GS:ffffffff8266b000(0000) knlGS:0000000000000000
[    4.540628] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    4.544593] CR2: ffff8c397ffff000 CR3: 0000000125020000 CR4: 00000000000406b0
[    4.549558] Call Trace:
[    4.551266]  apply_wqattrs_prepare+0x154/0x280
[    4.554357]  apply_workqueue_attrs_locked+0x4e/0xe0
[    4.557728]  apply_workqueue_attrs+0x36/0x60
[    4.560654]  alloc_workqueue+0x25a/0x6d0
[    4.563381]  ? kmem_cache_alloc_trace+0x1e3/0x500
[    4.566628]  ? __mutex_unlock_slowpath+0x44/0x3f0
[    4.569875]  workqueue_init_early+0x246/0x348
[    4.573025]  start_kernel+0x3c7/0x7ec
[    4.575558]  x86_64_start_reservations+0x40/0x49
[    4.578738]  x86_64_start_kernel+0xda/0xe4
[    4.581600]  secondary_startup_64+0xb6/0xc0
[    4.584473] Modules linked in:
[    4.586620] ---[ end trace f67eb9af4d8d492b ]---

Fix this by ensuring the value we set with set_freepointer is either NULL
or another value in the chain.

Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: Laura Abbott <labbott@redhat.com>
---
 mm/slub.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index e6c030e47364..8834563cdb4b 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1432,7 +1432,9 @@ static inline bool slab_free_freelist_hook(struct kmem_cache *s,
 	void *old_tail = *tail ? *tail : *head;
 	int rsize;
 
-	if (slab_want_init_on_free(s))
+	if (slab_want_init_on_free(s)) {
+		void *p = NULL;
+
 		do {
 			object = next;
 			next = get_freepointer(s, object);
@@ -1445,8 +1447,10 @@ static inline bool slab_free_freelist_hook(struct kmem_cache *s,
 							   : 0;
 			memset((char *)object + s->inuse, 0,
 			       s->size - s->inuse - rsize);
-			set_freepointer(s, object, next);
+			set_freepointer(s, object, p);
+			p = object;
 		} while (object != old_tail);
+	}
 
 /*
  * Compiler cannot detect this function can be removed if slab_free_hook()
-- 
2.21.0