CVE-2026-89651ceph: bound MDSCapAuth path and fs_name decode in handle_session()

Official description

In the Linux kernel, the following vulnerability has been resolved: ceph: bound MDSCapAuth path and fs_name decode in handle_session() handle_session() decodes the MDSCapAuth records carried by a CEPH_SESSION_OPEN 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 ceph_decode_copy(). Unlike the surrounding fields, which all use the _safe decode variants, these two copies are not preceded by a ceph_decode_need() 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 ceph_decode_copy() read past the end of the front buffer. The message front is a dedicated allocation (ceph_msg_new2() -> 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 ceph_decode_copy_safe(), which performs the ceph_decode_need() 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.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/dc5c7dffda92ef372fa587c444a1c537373df824
• https://git.kernel.org/stable/c/c37db86d2b5e9b57001ccae7e7a727e95a4f9c02
• https://git.kernel.org/stable/c/5f892c767b30e4fb0c744c59f9235c717383d2cd
• https://git.kernel.org/stable/c/77933e22adfe813be2bd10be08d6e950103c3967