CVE-2026-80557

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

libceph: fix OOB read in decode_watchers() via missing bounds check

cephstartdecoding() validates that struct_len bytes remain in the

buffer after the encoding header, but accepts struct_len=0 as valid:

cephdecodeneed(p, end, 0, bad) always passes. When a malicious or

compromised OSD sends an objlistwatchresponset reply with

structlen=0, cephstart_decoding() returns success with p == end,

leaving zero bytes guaranteed for subsequent reads.

The immediately following cephdecode32(p) in decode_watchers() has

no preceding bounds check. With p == end this is a 4-byte read past

the validated buffer boundary. The garbage value is then passed

directly to kzalloc_objs() as the watcher count.

The sibling function decode_watcher() already uses the safe variants

(cephdecodecopysafe, cephdecode64safe, cephdecodeskip_32)

after its own cephstartdecoding() call. decode_watchers() is the

only site that uses the bare variant, confirming an oversight.

Fix by replacing cephdecode32(p) with cephdecode32_safe(p, end,

*num_watchers, bad), consistent with the established pattern.

Attacker model: a malicious or compromised OSD in a multi-tenant Ceph

deployment (e.g. cloud) can trigger this against any kernel client

that calls CEPHOSDOPLISTWATCHERS, without any further privileges

beyond OSD session establishment.

[ idryomov: trim changelog ]