libceph: validate banner payload length

When parsing the Ceph messenger v2 protocol banner, the `payload_len` field
is decoded from the banner prefix. If a client sends a banner with a
`payload_len` of 0, the kernel sets up a 0-length socket read. This
violates an invariant in the state machine, triggering a warning in
`populate_in_iter()`:

------------[ cut here ]------------
!iov_iter_count(&con->v2.in_iter)
WARNING: net/ceph/messenger_v2.c:3129 at populate_in_iter
net/ceph/messenger_v2.c:3129 [inline], CPU#1: kworker/1:3/5070
WARNING: net/ceph/messenger_v2.c:3129 at ceph_con_v2_try_read+0x6634/0x6810
net/ceph/messenger_v2.c:3159, CPU#1: kworker/1:3/5070
...
Call Trace:
 <TASK>
 ceph_con_workfn+0x1f5/0x14a0 net/ceph/messenger.c:1575
 process_one_work kernel/workqueue.c:3322 [inline]
 process_scheduled_works+0xa8e/0x14e0 kernel/workqueue.c:3405
 worker_thread+0xa47/0xfb0 kernel/workqueue.c:3486
 kthread+0x388/0x470 kernel/kthread.c:436
 ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>

According to the msgr2 protocol specification, the banner payload is
expected to contain at least two 64-bit integers (`server_feat` and
`server_req_feat`). Therefore, `payload_len` must be at least 16 bytes.

Fix this by adding a check in `process_banner_prefix()` to reject a
`payload_len` smaller than 16 bytes. This prevents the 0-length read and
correctly aborts the connection with a protocol error.

Fixes: cd1a677cad ("libceph, ceph: implement msgr2.1 protocol (crc and secure modes)")
Assisted-by: Gemini:gemini-3.5-flash Gemini:gemini-3.1-pro-preview syzbot
Reported-by: syzbot+87c7c2d63c44e41c77a3@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=87c7c2d63c44e41c77a3
Link: https://syzkaller.appspot.com/ai_job?id=c8ca3d63-717a-4933-89ec-f3d761b8690d
Signed-off-by: Aleksandr Nogikh <nogikh@google.com>
Reviewed-by: Alex Markuze <amarkuze@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

net/ceph/messenger_v2.c

@@ -2142,6 +2142,11 @@ static int process_banner_prefix(struct ceph_connection *con)
 	payload_len = ceph_decode_16(&p);
 	dout("%s con %p payload_len %d\n", __func__, con, payload_len);
 
+	if (payload_len < sizeof(u64) + sizeof(u64)) {
+		con->error_msg = "protocol error, bad banner payload len";
+		return -EINVAL;
+	}
+
 	return prepare_read_banner_payload(con, payload_len);
 }