mm/slub: avoid zeroing outside-object freepointer for single free

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