Messages in this thread | | | Date | Mon, 29 Apr 2024 09:55:55 +0200 | Subject | Re: [PATCH] slub: Fixes freepointer encoding for single free | From | Nicolas Bouchinet <> |
| |
On 4/26/24 15:14, Xiongwei Song wrote: > On Fri, Apr 26, 2024 at 8:18 PM Nicolas Bouchinet > <nicolas.bouchinet@clip-os.org> wrote: >> On 4/26/24 11:20, Xiongwei Song wrote: >>> On Wed, Apr 24, 2024 at 8:48 PM Nicolas Bouchinet >>> <nicolas.bouchinet@clip-os.org> wrote: >>>> 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 >>> 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 >
| |