[PATCH net] net: rose: reject truncated CLEAR_REQUEST frames in state machines

[Mashiro Chen <mashiro.chen@mailbox.org>]

All five ROSE state machines (states 1-5) handle ROSE_CLEAR_REQUEST
by reading the cause and diagnostic bytes directly from skb->data[3]
and skb->data[4] without verifying that the frame is long enough:

  rose_disconnect(sk, ..., skb->data[3], skb->data[4]);

The entry-point check in rose_route_frame() only enforces
ROSE_MIN_LEN (3 bytes), so a remote peer on a ROSE network can
send a syntactically valid but truncated CLEAR_REQUEST (3 or 4
bytes) while a connection is open in any state.  Processing such a
frame causes a one- or two-byte out-of-bounds read past the skb
data, leaking uninitialized heap content as the cause/diagnostic
values returned to user space via getsockopt(ROSE_GETCAUSE).

Add a single length check at the rose_process_rx_frame() dispatch
point, before any state machine is entered, to drop frames that
carry the CLEAR_REQUEST type code but are too short to contain the
required cause and diagnostic fields.

Cc: stable@vger.kernel.org
Cc: linux-hams@vger.kernel.org
Signed-off-by: Mashiro Chen <mashiro.chen@mailbox.org>
---
 net/rose/rose_in.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/rose/rose_in.c b/net/rose/rose_in.c
index 0276b393f0e530..e2680058196273 100644
--- a/net/rose/rose_in.c
+++ b/net/rose/rose_in.c
@@ -271,6 +271,13 @@ int rose_process_rx_frame(struct sock *sk, struct sk_buff *skb)
 
 	frametype = rose_decode(skb, &ns, &nr, &q, &d, &m);
 
+	/*
+	 * ROSE_CLEAR_REQUEST carries cause and diagnostic in bytes 3..4.
+	 * Reject a malformed frame that is too short to contain them.
+	 */
+	if (frametype == ROSE_CLEAR_REQUEST && skb->len < 5)
+		return 0;
+
 	switch (rose->state) {
 	case ROSE_STATE_1:
 		queued = rose_state1_machine(sk, skb, frametype);
-- 
2.53.0