CVE-2026-89650

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

ceph: bound numexporttargets array for mds info v2/v3

cephmdsmapdecode() in fs/ceph/mdsmap.c reads numexporttargets from

each per-mds info record and advances the decode cursor by

numexporttargets * sizeof(u32) without first checking that many bytes

remain. The only upper-bound check that catches a runaway cursor

(*p > infoend) is gated on infov >= 4, because info_end is left NULL

for info_v 2 and 3. When the monitor sends an MDS map whose per-mds

info version is 2 or 3 with an oversized numexporttargets, the cursor

moves past the message front buffer and the later export-targets loop

calls the unchecked cephdecode32() on out-of-bounds memory.

A kernel client processes CEPHMSGMDS_MAP from its monitor session

(net/ceph/mon_client.c dispatches it; fs/ceph/super.c routes it to

cephmdschandle_mdsmap(), which sets end to the front buffer bound and

calls cephmdsmapdecode()). A malicious or compromised monitor, or an

on-path attacker on an unsigned/unencrypted messenger session, can

therefore drive an out-of-bounds read in the client kernel; on x86_64

with KASAN it is reported as a slab-out-of-bounds read in

cephmdsmapdecode(). The decoded values land in the internal

info->export_targets[] array, so the consequence is a kernel

out-of-bounds read, not an information leak to the attacker.

Impact: a malicious or compromised Ceph monitor sending an MDS map with

a per-mds info version of 2 or 3 and an oversized numexporttargets

field triggers an out-of-bounds read in the CephFS client kernel.

Add a cephdecodeneed() for the export-targets array before advancing

the cursor, so the bound is enforced for every info_v >= 2, not only

info_v >= 4. This mirrors the count-then-need idiom already used for

mdatapg_pools later in the same function.

Compute the export-targets byte count with size_mul() and reuse that

checked length when advancing the cursor, so the attacker-controlled

numexporttargets multiplication fails closed on overflow rather than

relying on the later kcalloc() guard.