[PATCH v3 3/4] ceph: bound num_export_targets array for mds info v2/v3

[Michael Bommarito <michael.bommarito@gmail.com>]

ceph_mdsmap_decode() in fs/ceph/mdsmap.c reads num_export_targets from
each per-mds info record and advances the decode cursor by
num_export_targets * sizeof(u32) without first checking that many bytes
remain. The only upper-bound check that catches a runaway cursor
(*p > info_end) is gated on info_v >= 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 num_export_targets, the cursor
moves past the message front buffer and the later export-targets loop
calls the unchecked ceph_decode_32() on out-of-bounds memory.

A kernel client processes CEPH_MSG_MDS_MAP from its monitor session
(net/ceph/mon_client.c dispatches it; fs/ceph/super.c routes it to
ceph_mdsc_handle_mdsmap(), which sets end to the front buffer bound and
calls ceph_mdsmap_decode()). 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
ceph_mdsmap_decode(). 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 num_export_targets
field triggers an out-of-bounds read in the CephFS client kernel.

Add a ceph_decode_need() 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
m_data_pg_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
num_export_targets multiplication fails closed on overflow rather than
relying on the later kcalloc() guard.

Fixes: d463a43d69f4 ("ceph: CEPH_FEATURE_MDSENC support")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Reviewed-by: Viacheslav Dubeyko <slava@dubeyko.com>
---
 fs/ceph/mdsmap.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/ceph/mdsmap.c b/fs/ceph/mdsmap.c
index d8e46eb7e5eb5..c0f63f0460f67 100644
--- a/fs/ceph/mdsmap.c
+++ b/fs/ceph/mdsmap.c
@@ -3,6 +3,7 @@
 
 #include <linux/bug.h>
 #include <linux/err.h>
+#include <linux/overflow.h>
 #include <linux/random.h>
 #include <linux/slab.h>
 #include <linux/types.h>
@@ -126,6 +127,7 @@ struct ceph_mdsmap *ceph_mdsmap_decode(struct ceph_mds_client *mdsc, void **p,
 	u8 mdsmap_v;
 	u16 mdsmap_ev;
 	u32 target;
+	size_t export_targets_len;
 
 	m = kzalloc_obj(*m, GFP_NOFS);
 	if (!m)
@@ -224,8 +226,11 @@ struct ceph_mdsmap *ceph_mdsmap_decode(struct ceph_mds_client *mdsc, void **p,
 		*p += namelen;
 		if (info_v >= 2) {
 			ceph_decode_32_safe(p, end, num_export_targets, bad);
+			export_targets_len = size_mul(num_export_targets,
+						      sizeof(u32));
+			ceph_decode_need(p, end, export_targets_len, bad);
 			pexport_targets = *p;
-			*p += num_export_targets * sizeof(u32);
+			*p += export_targets_len;
 		} else {
 			num_export_targets = 0;
 		}
-- 
2.53.0