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CVE-2021-47221
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Description
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.)
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