In the Linux kernel, the following vulnerability has been resolved:
mm/slub: avoid zeroing outside-object freepointer for single free
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().
During bulk free, slab_free_freelist_hook() isn’t affected as it always
sets it objects freepointer using set_freepointer() to maintain its
reconstructed freelist after init_on_free.
For single free, object’s freepointer thus needs to be avoided when
stored outside the object if init_on_free is set. The freepointer left
as is, check_object() may later detect an invalid pointer value due to
objects overflow.
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
[ 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