CVE-2021-47221

In the Linux kernel, the following vulnerability has been resolved:

mm/slub: actually fix freelist pointer vs redzoning

It turns out that SLUB redzoning (slub_debug=Z) checks from s->object_size rather than from s->inuse (which is normally bumped to make room for the freelist pointer), so a cache created with an object size less than 24 would have the freelist pointer written beyond s->object_size, causing the redzone to be corrupted by the freelist pointer. This was very visible with slub_debug=ZF:

BUG test (Tainted: G B ): Right Redzone overwritten

INFO: 0xffff957ead1c05de-0xffff957ead1c05df @offset=1502. First byte 0x1a instead of 0xbb INFO: Slab 0xffffef3950b47000 objects=170 used=170 fp=0x0000000000000000 flags=0x8000000000000200 INFO: Object 0xffff957ead1c05d8 @offset=1496 fp=0xffff957ead1c0620

Redzone (ptrval): bb bb bb bb bb bb bb bb …….. Object (ptrval): 00 00 00 00 00 f6 f4 a5 …….. Redzone (ptrval): 40 1d e8 1a aa @…. Padding (ptrval): 00 00 00 00 00 00 00 00 ……..

Adjust the offset to stay within s->object_size.

(Note that no caches of in this size range are known to exist in the kernel currently.)

Affected Software

References

• https://git.kernel.org/stable/c/ce6e8bee7a3883e8008b30f5887dbb426aac6a35
• https://git.kernel.org/stable/c/e41a49fadbc80b60b48d3c095d9e2ee7ef7c9a8e
• https://git.kernel.org/stable/c/f6ed2357541612a13a5841b3af4dc32ed984a25f