CVE-2026-68082: libceph: fix two unsafe bare decodes in decode_lockers()

From: Greg Kroah-Hartman <[email protected]>

Description
===========

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

libceph: fix two unsafe bare decodes in decode_lockers()

decode_lockers() in cls_lock_client.c contains two bare decode operations
that allow a malicious or compromised OSD to trigger slab-out-of-bounds
reads:

1. ceph_decode_32(p) at the num_lockers field has no preceding bounds
   check. ceph_start_decoding() accepts struct_len=0 as valid -- the
   internal ceph_decode_need(p, end, 0, bad) always passes -- so when an
   OSD sends struct_len=0, ceph_start_decoding() returns success with
   p == end. The immediately following bare ceph_decode_32(p) then reads
   4 bytes past the validated buffer boundary. The garbage value is
   passed directly to kzalloc_objs() as the locker count.

   The sibling function decode_watchers() in osd_client.c already uses
   ceph_decode_32_safe() after its own ceph_start_decoding() call.
   decode_lockers() was the only site using the bare variant.

2. ceph_decode_8(p) after the decode_locker() loop has no preceding
   bounds check. If an OSD crafts num_lockers such that the loop
   advances p exactly to end, the subsequent bare ceph_decode_8(p) reads
   one byte past the validated buffer boundary. The result is passed
   directly into *type, which is used as a lock type discriminator by
   callers, giving an OSD-controlled one-byte OOB read with direct
   influence over the lock type field.

Fix both by replacing bare operations with their safe variants:
  ceph_decode_32(p) -> ceph_decode_32_safe(p, end, *num_lockers,
                                           err_inval)
  ceph_decode_8(p)  -> ceph_decode_8_safe(p, end, *type,
                                          err_free_lockers)

The goto targets differ intentionally:
  err_inval: is a new label returning -EINVAL directly. It is used for
  the pre-allocation failure path where *lockers is not yet allocated
  and must not be passed to ceph_free_lockers().

  err_free_lockers: is the existing label. It is used for the
  post-allocation failure path where *lockers is allocated and must
  be freed.

ret is set to -EINVAL before ceph_decode_8_safe() so that
err_free_lockers returns the correct error code on bounds violation.
Without this, err_free_lockers would return a stale ret value (0 from
the successful decode_locker() loop), silently swallowing the error.

-EINVAL is correct for both failure paths. The data received from the
OSD is structurally malformed. -ENOMEM would misrepresent the failure
class to callers and to stable@ backporters triaging error paths.

Attacker model: a malicious or compromised OSD in a multi-tenant Ceph
deployment can trigger this against any kernel client that issues the
lock.get_info class method (e.g. during RBD exclusive lock acquisition).

[ idryomov: trim changelog, formatting ]

The Linux kernel CVE team has assigned CVE-2026-68082 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 4.9 with commit d4ed4a530562881cc5225050e42d96034f405aae and fixed in 7.1.6 with commit a54be593d0b749161b08a1e56189b2cb9114267a
	Issue introduced in 4.9 with commit d4ed4a530562881cc5225050e42d96034f405aae and fixed in 7.2-rc5 with commit a109a556115271ca7896dcda7b4b7e45e156c227

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-68082
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	net/ceph/cls_lock_client.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/a54be593d0b749161b08a1e56189b2cb9114267a
	https://git.kernel.org/stable/c/a109a556115271ca7896dcda7b4b7e45e156c227