UBUNTU-CVE-2026-89651: Critical severity ubuntu/linux-hwe-edge vulnerability

In the Linux kernel, the following vulnerability has been resolved: ceph: bound MDSCapAuth path and fsname decode in handlesession() handlesession() decodes the MDSCapAuth records carried by a CEPHSESSIONOPEN message (msgversion >= 6). For each record the match.path and match.fsname 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 structlen 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 handlesession(). 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 cephdecodecopysafe(), 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 capauths array.