On 4/26/24 11:20, Xiongwei Song wrote:
On Wed, Apr 24, 2024 at 8:48 PM Nicolas Bouchinet
<nicolas.bouchinet@xxxxxxxxxxx> wrote:
From: Nicolas Bouchinet <nicolas.bouchinet@xxxxxxxxxxx>
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
If init_on_free is set, slab_free_hook() zeros the object first, then
do_slab_free() calls
set_freepointer() to set the fp value, so there are 8 bytes non-zero
at the moment?
Hence, the issue is not related to init_on_free?
The fp=0x7ee4f480ce0ecd7c here is beyond kernel memory space, is the issue from
CONFIG_SLAB_FREELIST_HARDENED enabled?
My understanding of the bug is that slab_free_hook() indeed zeroes the
object and its metadata first, then calls do_slab_free() and directly
calls __slab_free(), head an tail parameters being set to the object.
If `slub_debug=F` (SLAB_CONSISTENCY_CHECKS) is set, the following call
path can be executed :
free_to_partial_list() ->
free_debug_processing() ->
free_consistency_checks() ->
check_object() ->
check_valid_pointer(get_freepointer())
When check_valid_pointer() is called, its object parameter is first
decoded by get_freepointer(), if CONFIG_SLAB_FREELIST_HARDENED is
enabled, zeroed data is then decoded by freelist_ptr_decode() using the
(ptr.v ^ s->random ^ swab(ptr_addr)) operation.
Does that makes sense to you or do you think I'm missing something ?
Best regards,
Nicolas
Xiongwei
[ 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@xxxxxxxxxxx>
---
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
