[PATCH -next] slub: play init_on_free=1 well with SLAB_RED_ZONE

[Qian Cai]

The linux-next commit "mm: security: introduce init_on_alloc=1 and
init_on_free=1 boot options" [1] does not play well with SLAB_RED_ZONE
as it will overwrite the right-side redzone with all zeros and triggers
endless errors below. Fix it by only wiping out the slab object size and
leave the redzone along. This has a side-effect that it does not wipe
out the slab object metadata like the free pointer and the tracking data
for SLAB_STORE_USER which does seem important anyway, so just to keep
the code simple.

[1] https://patchwork.kernel.org/patch/10999465/

BUG kmalloc-64 (Tainted: G    B            ): Redzone overwritten

INFO: 0x(____ptrval____)-0x(____ptrval____). First byte 0x0 instead of
0xcc
INFO: Slab 0x(____ptrval____) objects=163 used=4 fp=0x(____ptrval____)
flags=0x3fffc000000201
INFO: Object 0x(____ptrval____) @offset=58008 fp=0x(____ptrval____)

Redzone (____ptrval____): cc cc cc cc cc cc cc cc
........
Object (____ptrval____): 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
................
Object (____ptrval____): 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
................
Object (____ptrval____): 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
................
Object (____ptrval____): 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
................
Redzone (____ptrval____): 00 00 00 00 00 00 00 00
........
Padding (____ptrval____): 00 00 00 00 00 00 00 00
........
CPU: 0 PID: 1 Comm: swapper/0 Tainted: G    B
5.2.0-rc5-next-20190620+ #2
Call Trace:
[c00000002b72f4b0] [c00000000089ce5c] dump_stack+0xb0/0xf4 (unreliable)
[c00000002b72f4f0] [c0000000003e13d8] print_trailer+0x23c/0x264
[c00000002b72f580] [c0000000003d0468] check_bytes_and_report+0x138/0x160
[c00000002b72f620] [c0000000003d33dc] check_object+0x2ac/0x3e0
[c00000002b72f690] [c0000000003da15c] free_debug_processing+0x1ec/0x680
[c00000002b72f780] [c0000000003da944] __slab_free+0x354/0x6d0
[c00000002b72f840] [c00000000015600c]
__kthread_create_on_node+0x15c/0x260
[c00000002b72f910] [c000000000156144] kthread_create_on_node+0x34/0x50
[c00000002b72f930] [c000000000146fd0] create_worker+0xf0/0x230
[c00000002b72f9e0] [c00000000014fc6c] workqueue_prepare_cpu+0xdc/0x280
[c00000002b72fa60] [c00000000011b27c] cpuhp_invoke_callback+0x1bc/0x1220
[c00000002b72fb00] [c00000000011e7d8] _cpu_up+0x168/0x340
[c00000002b72fb80] [c00000000011eafc] do_cpu_up+0x14c/0x210
[c00000002b72fc10] [c000000000aedc90] smp_init+0x17c/0x1f0
[c00000002b72fcb0] [c000000000ac4a4c] kernel_init_freeable+0x358/0x7cc
[c00000002b72fdb0] [c0000000000106ec] kernel_init+0x2c/0x150
[c00000002b72fe20] [c00000000000b4cc] ret_from_kernel_thread+0x5c/0x70
FIX kmalloc-64: Restoring 0x(____ptrval____)-0x(____ptrval____)=0xcc

FIX kmalloc-64: Object at 0x(____ptrval____) not freed

Signed-off-by: Qian Cai <cai@xxxxxx>
---
 mm/slub.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index a384228ff6d3..787971d4fa36 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1437,7 +1437,7 @@ static inline bool slab_free_freelist_hook(struct kmem_cache *s,
 		do {
 			object = next;
 			next = get_freepointer(s, object);
-			memset(object, 0, s->size);
+			memset(object, 0, s->object_size);
 			set_freepointer(s, object, next);
 		} while (object != old_tail);
 
-- 
1.8.3.1