Monitor and MDS messages can be allocated with only front-buffer storage
and no data items.  The messenger receive path copies the wire header
into the selected ceph_msg and later uses hdr.data_len to decide whether
to initialize a data cursor.

If a malicious or compromised peer advertises a non-zero data segment for
one of these front-only messages, the receive path can call
ceph_msg_data_cursor_init() with length greater than msg->data_length and
hit its BUG_ON() checks, crashing the client kernel.

I verified the monitor issue against v7.1-rc1-123-ge75a43c7cec4.  The
msgr2 trigger path is present since commit cd1a677cad99 ("libceph,
ceph: implement msgr2.1 protocol (crc and secure modes)"), which is
contained in v5.11-rc1 and later.  The allocator patterns are older, but
I have not tested older msgr1-only kernels.

A concrete monitor trigger is a monitor connection over msgr2 after
CEPH_CON_S_OPEN where a FRAME_TAG_MESSAGE contains a monitor reply type
handled by mon_alloc_msg(), a valid front_len for that message type and
data_len = 1.  CEPH_MSG_MON_MAP is one such example: the message is
allocated with ceph_msg_new(), leaving msg->data_length and
msg->num_data_items as zero.

The MDS allocation path has the same issue: mds_alloc_msg() allocates the
incoming message with ceph_msg_new(type, front_len, GFP_NOFS, false),
leaving msg->data_length as zero for a front-only message selected from
an attacker-controlled wire header.

Reject monitor and MDS messages whose wire data segment is larger than
the data backing allocated for the selected ceph_msg, mirroring the
existing OSD reply hardening.

Fixes: 53ded495c6ac ("libceph: define mds_alloc_msg() method")
Fixes: cd1a677cad99 ("libceph, ceph: implement msgr2.1 protocol (crc and secure modes)")
Signed-off-by: Dhiraj Mishra <mishra.dhiraj95@gmail.com>
---
v4:
  - Add the same oversized data_len guard to the MDS allocation path.

v3:
  - Remove the impossible !req->reply check for generic requests.
  - Use pr_warn_ratelimited() for malicious-monitor log spam resistance.
  - Avoid adding __func__ to the new mon_client warnings.
  - Add a blank line between the front_len and data_len checks.

v2:
  - Resend as an inline plain-text patch.
  - Add full email address to the From and Signed-off-by identities.
  - Add ceph-devel and LKML to the recipient list when sending.

 fs/ceph/mds_client.c  | 13 +++++++++++++
 net/ceph/mon_client.c | 17 +++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
index ed17e0023705..3b92d37983d1 100644
--- a/fs/ceph/mds_client.c
+++ b/fs/ceph/mds_client.c
@@ -6543,9 +6543,12 @@ static int mds_handle_auth_bad_method(struct ceph_connection *con,
 static struct ceph_msg *mds_alloc_msg(struct ceph_connection *con,
 				struct ceph_msg_header *hdr, int *skip)
 {
+	struct ceph_mds_session *s = con->private;
+	struct ceph_client *cl = s->s_mdsc->fsc->client;
 	struct ceph_msg *msg;
 	int type = (int) le16_to_cpu(hdr->type);
 	int front_len = (int) le32_to_cpu(hdr->front_len);
+	u32 data_len = le32_to_cpu(hdr->data_len);
 
 	if (con->in_msg)
 		return con->in_msg;
@@ -6558,6 +6561,16 @@ static struct ceph_msg *mds_alloc_msg(struct ceph_connection *con,
 		return NULL;
 	}
 
+	if (data_len > msg->data_length) {
+		pr_warn_ratelimited_client(cl,
+					   "mds%d message data %u > prealloc %zu, skipping\n",
+					   s->s_mds, data_len,
+					   msg->data_length);
+		ceph_msg_put(msg);
+		*skip = 1;
+		return NULL;
+	}
+
 	return msg;
 }
 
diff --git a/net/ceph/mon_client.c b/net/ceph/mon_client.c
index d2cdc8ee3155..9f1c7ca42f36 100644
--- a/net/ceph/mon_client.c
+++ b/net/ceph/mon_client.c
@@ -712,6 +712,7 @@ static struct ceph_msg *get_generic_reply(struct ceph_connection *con,
 	struct ceph_mon_client *monc = con->private;
 	struct ceph_mon_generic_request *req;
 	u64 tid = le64_to_cpu(hdr->tid);
+	u32 data_len = le32_to_cpu(hdr->data_len);
 	struct ceph_msg *m;
 
 	mutex_lock(&monc->mutex);
@@ -720,6 +721,11 @@ static struct ceph_msg *get_generic_reply(struct ceph_connection *con,
 		dout("get_generic_reply %lld dne\n", tid);
 		*skip = 1;
 		m = NULL;
+	} else if (data_len > req->reply->data_length) {
+		pr_warn_ratelimited("mon generic reply tid %llu data %u > preallocated %zu, skipping\n",
+				    tid, data_len, req->reply->data_length);
+		*skip = 1;
+		m = NULL;
 	} else {
 		dout("get_generic_reply %lld got %p\n", tid, req->reply);
 		*skip = 0;
@@ -1499,6 +1505,7 @@ static struct ceph_msg *mon_alloc_msg(struct ceph_connection *con,
 	struct ceph_mon_client *monc = con->private;
 	int type = le16_to_cpu(hdr->type);
 	int front_len = le32_to_cpu(hdr->front_len);
+	u32 data_len = le32_to_cpu(hdr->data_len);
 	struct ceph_msg *m = NULL;
 
 	*skip = 0;
@@ -1545,5 +1552,15 @@ static struct ceph_msg *mon_alloc_msg(struct ceph_connection *con,
 		m = ceph_msg_new(type, front_len, GFP_NOFS, false);
 	}
 
+	if (m && data_len > m->data_length) {
+		pr_warn_ratelimited("mon message data %u > prealloc %zu (%u#%llu), skipping\n",
+				    data_len, m->data_length,
+				    (unsigned int)con->peer_name.type,
+				    le64_to_cpu(con->peer_name.num));
+		ceph_msg_put(m);
+		m = NULL;
+		*skip = 1;
+	}
+
 	return m;
 }