CVE-2026-89651

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

ceph: bound MDSCapAuth path and fsname decode in handlesession()

handle_session() decodes the MDSCapAuth records carried by a

CEPHSESSIONOPEN message (msg_version >= 6). For each record the

match.path and match.fs_name byte strings are read by first decoding a

32-bit length and then copying that many bytes with the bare

cephdecodecopy(). Unlike the surrounding fields, which all use the

_safe decode variants, these two copies are not preceded by a

cephdecodeneed() bounds check, and the enclosing MDSCapAuth and

MDSCapMatch struct_len fields are skipped rather than enforced as an

upper bound. A length larger than the bytes remaining in the message

front makes cephdecodecopy() read past the end of the front buffer.

The message front is a dedicated allocation (cephmsgnew2() ->

kvmalloc), so the over-read runs off that object. A malicious or

compromised MDS can trigger this with the first post-connect message on

mount, with no client-side user interaction; under KASAN it is reported

as a slab-out-of-bounds read in handle_session().

Impact: a malicious MDS can force the kernel client to read up to 4 GiB

past the message front allocation during session setup, crashing the

client (out-of-bounds read).

Switch both copies to cephdecodecopy_safe(), which performs the

cephdecodeneed() bounds check before the copy and branches to the

existing bad label, matching the rest of the decoder and the error path

that frees the partially decoded cap_auths array.