caif: fix integer underflow in cffrml_receive()

The cffrml_receive() function extracts a length field from the packet
header and, when FCS is disabled, subtracts 2 from this length without
validating that len >= 2.

If an attacker sends a malicious packet with a length field of 0 or 1
to an interface with FCS disabled, the subtraction causes an integer
underflow.

This can lead to memory exhaustion and kernel instability, potential
information disclosure if padding contains uninitialized kernel memory.

Fix this by validating that len >= 2 before performing the subtraction.

Reported-by: Yuhao Jiang <[e-mail]>
Reported-by: Junrui Luo <[e-mail]>
Fixes: b482cd2053e3 ("net-caif: add CAIF core protocol stack")
Signed-off-by: Junrui Luo <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/caif/cffrml.c

···
 	len = le16_to_cpu(tmp);
 
 	/* Subtract for FCS on length if FCS is not used. */
-	if (!this->dofcs)
+	if (!this->dofcs) {
+		if (len < 2) {
+			++cffrml_rcv_error;
+			pr_err("Invalid frame length (%d)\n", len);
+			cfpkt_destroy(pkt);
+			return -EPROTO;
+		}
 		len -= 2;
+	}
 
 	if (cfpkt_setlen(pkt, len) < 0) {
 		++cffrml_rcv_error;