On 4/26/24 15:14, Xiongwei Song wrote:
On Fri, Apr 26, 2024 at 8:18 PM Nicolas Bouchinet
<nicolas.bouchinet@xxxxxxxxxxx> wrote:
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())
I understand the call path. I meant here the freepointer is not ZERO
but an illegal
value( fp=0x7ee4f480ce0ecd7c),
Yes this is the reason of this patch. The freepointer is an illegal
value because the memory range where it sits has been overridden by
zeroes, set_freepointer() is never called and thus the freepointer is
never properly set.
The illegal value is obtained after zeroes has been decoded by
get_freepointer()->freelist_ptr_decode().
then check_valid_pointer return 1 with
it's last line
and then check_object() printed out the error message. I'm not sure if I
misunderstood you.
check_valid_pointer() returns 0 since object < base, the object being
the decoded fp (0x7ee4f480ce0ecd7c < 0xffff.* base addr), hence
check_object() returns 0, not 1.
This is why the "Object at 0x%p not freed" slab_fix() is called in
free_debug_processing().
Thank,
Xiongwei
