[PATCH v3 net] rose: fix OOB reads on short CLEAR REQUEST frames

[Ashutosh Desai <ashutoshdesai993@gmail.com>]

rose_process_rx_frame() calls rose_decode() which reads skb->data[2]
without any prior length check. For CLEAR REQUEST frames the state
machines then read skb->data[3] and skb->data[4] as the cause and
diagnostic bytes.

A crafted 3-byte ROSE CLEAR REQUEST frame passes the minimum length
gate in rose_route_frame() and reaches rose_process_rx_frame(), where
rose_decode() reads one byte past the header and the state machines
read two bytes past the valid buffer. A remote peer can exploit this
to leak kernel memory contents or trigger a kernel panic.

Add a pskb_may_pull(skb, 3) check before rose_decode() to cover its
skb->data[2] access, and a pskb_may_pull(skb, 5) check afterwards for
the CLEAR REQUEST path to cover the cause and diagnostic reads.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Ashutosh Desai <ashutoshdesai993@gmail.com>
---
V2 -> V3: drop kfree_skb() calls to fix double-free; add end-user
          visible symptom to commit log; use [net] subject prefix
V1 -> V2: switch skb->len check to pskb_may_pull; add pskb_may_pull(skb, 3)
          before rose_decode() to cover its skb->data[2] access

v2: https://lore.kernel.org/netdev/177614667427.3606651.8700070406932922261@gmail.com/
v1: https://lore.kernel.org/netdev/20260409013246.2051746-1-ashutoshdesai993@gmail.com/

 net/rose/rose_in.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/rose/rose_in.c b/net/rose/rose_in.c
index 0276b393f0e5..8e60dc562b4a 100644
--- a/net/rose/rose_in.c
+++ b/net/rose/rose_in.c
@@ -269,8 +269,14 @@ int rose_process_rx_frame(struct sock *sk, struct sk_buff *skb)
 	if (rose->state == ROSE_STATE_0)
 		return 0;
 
+	if (!pskb_may_pull(skb, 3))
+		return 0;
+
 	frametype = rose_decode(skb, &ns, &nr, &q, &d, &m);
 
+	if (frametype == ROSE_CLEAR_REQUEST && !pskb_may_pull(skb, 5))
+		return 0;
+
 	switch (rose->state) {
 	case ROSE_STATE_1:
 		queued = rose_state1_machine(sk, skb, frametype);
-- 
2.34.1