Use After Free Affecting kernel-rt package, versions *

Note: Versions mentioned in the description apply only to the upstream kernel-rt package and not the kernel-rt package as distributed by RHEL. See How to fix? for RHEL:7 relevant fixed versions and status.

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

• https://access.redhat.com/security/cve/CVE-2024-36892
• https://git.kernel.org/stable/c/56900355485f6e82114b18c812edd57fd7970dcb
• https://git.kernel.org/stable/c/8f828aa48812ced28aa39cb3cfe55ef2444d03dd