[PATCH] ocfs2/dlm: validate lockname_len in dlm_mig_lockres_handler()

[Ibrahim Hashimov <security@auditcode.ai>]

dlm_mig_lockres_handler() handles an incoming DLM_MIG_LOCKRES_MSG o2net
message from a remote DLM domain node and copies the migratable lockres
straight out of the wire buffer:

    struct dlm_migratable_lockres *mres =
        (struct dlm_migratable_lockres *)msg->buf;
    ...
    res = dlm_new_lockres(dlm, mres->lockname, mres->lockname_len);

mres->lockname_len is a raw u8 field taken verbatim from the network
message (dlmcommon.h), so it can be anywhere in [0, 255].
dlm_new_lockres() forwards it unchanged into dlm_init_lockres():

    qname = (char *) res->lockname.name;
    memcpy(qname, name, namelen);

res->lockname.name is allocated from dlm_lockname_cache, a dedicated
kmem_cache sized exactly DLM_LOCKID_NAME_MAX (32) bytes
(dlm_init_master_caches(), dlmmaster.c). Any lockname_len above 32
therefore makes memcpy() write past the end of that 32-byte slab object;
the maximum attacker-controlled value of 255 yields a 223-byte
slab-out-of-bounds write (CWE-787) with fully attacker-controlled content
(the bytes are mres->lockname itself).

The o2net receive path only bounds-checks the whole payload against
nh_max_len (cluster/tcp.c); it has no notion of the lockname_len
sub-field and cannot catch this. Every other DLM message handler that
consumes an attacker-supplied name length already guards it against
DLM_LOCKID_NAME_MAX right after pulling it out of the wire structure:
dlm_master_request_handler(), dlm_assert_master_handler(),
dlm_deref_lockres_handler() and dlm_deref_lockres_done_handler()
(dlmmaster.c), and the equivalent check in dlmconvert.c / dlmast.c.
dlm_mig_lockres_handler() is simply missing the same check -- present
since the file's introduction and never covered because exercising it
requires two heartbeating o2cb cluster nodes and a joined DLM domain,
i.e. it is invisible to single-node testing and to fuzzers that don't
speak the o2net/o2cb protocol.

The attacker is any node it fully controls (or is on-path and can spoof)
that is heartbeating in the same o2cb cluster and has been permitted to
join the DLM domain. o2net authentication is limited to cluster
membership (nodes list + heartbeat), not per-message payload validation,
so a single rogue or compromised cluster member, or an on-path attacker
able to inject on the unauthenticated o2net TCP stream, can send one
crafted DLM_MIG_LOCKRES_MSG frame with lockname_len = 255 to any other
domain member and corrupt adjacent dlm_lockname_cache slab objects,
leading to memory corruption and, depending on slab layout, a crash or
worse.

Reject the message before it reaches dlm_new_lockres() /
__dlm_lookup_lockres_full() if mres->lockname_len exceeds
DLM_LOCKID_NAME_MAX, mirroring the validation idiom already used by the
sibling handlers above. The check is placed immediately after the
existing dlm_joined() gate, before any use of mres->lockname /
mres->lockname_len, and follows this function's own early-return style
(dlm_put(dlm); return -EINVAL;) rather than the later "leave:" label,
since at this point buf/item are still NULL and no refs have been taken.
This is the smallest possible fix: one bounds check, no change to wire
format, on-disk format, or any other code path.

Reproduced on a v6.19 KASAN-instrumented kernel with an in-kernel
injector that calls dlm_mig_lockres_handler() directly (bypassing the
o2net accept/heartbeat gate that otherwise requires a second heartbeating
cluster node) against a live single-node OCFS2/o2cb DLM domain. A control
frame with lockname_len = 8 completed cleanly; an otherwise-identical
frame differing only in lockname_len = 255 reliably produced
"KASAN: slab-out-of-bounds in dlm_new_lockres", deterministically and
with no dependence on timing or allocator state. Full network delivery
over a real two-node o2cb cluster was not exercised: the injector proves
handler-reachability and the write is deterministic once the handler
runs, but the o2net wire path itself was not verified end-to-end.

Fixes: 6714d8e86bf4 ("[PATCH] OCFS2: The Second Oracle Cluster Filesystem")
Cc: stable@vger.kernel.org
Signed-off-by: Ibrahim Hashimov <security@auditcode.ai>
Assisted-by: AuditCode-AI:2026.07
---
 fs/ocfs2/dlm/dlmrecovery.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/ocfs2/dlm/dlmrecovery.c b/fs/ocfs2/dlm/dlmrecovery.c
index 9b97bf73df22..ec7edffc8ca8 100644
--- a/fs/ocfs2/dlm/dlmrecovery.c
+++ b/fs/ocfs2/dlm/dlmrecovery.c
@@ -1366,6 +1366,12 @@ int dlm_mig_lockres_handler(struct o2net_msg *msg, u32 len, void *data,
 		return -EINVAL;
 	}
 
+	if (mres->lockname_len > DLM_LOCKID_NAME_MAX) {
+		mlog(ML_ERROR, "Invalid name length!");
+		dlm_put(dlm);
+		return -EINVAL;
+	}
+
 	BUG_ON(!(mres->flags & (DLM_MRES_RECOVERY|DLM_MRES_MIGRATION)));
 
 	real_master = mres->master;
-- 
2.50.1 (Apple Git-155)