[PATCH net v3 4/4] nfc: llcp: fix OOB read of DM reason byte in nfc_llcp_recv_dm

["Lekë Hapçiu" <snowwlake@icloud.com>]

From: Lekë Hapçiu <framemain@outlook.com>

nfc_llcp_recv_dm() reads skb->data[2] (the DM reason byte) without
first verifying that skb->len is at least LLCP_HEADER_SIZE + 1.  A DM
PDU carrying only the 2-byte LLCP header from a rogue peer therefore
triggers a 1-byte OOB read.

Add the minimum-length guard at function entry, matching the pattern
used by nfc_llcp_recv_snl() and nfc_llcp_recv_agf().

Reachable from any NFC peer within ~4 cm once an LLCP link is up.

Fixes: d646960f7986 ("NFC: Add LLCP sockets")
Cc: stable@vger.kernel.org
Signed-off-by: Lekë Hapçiu <framemain@outlook.com>
---
 net/nfc/llcp_core.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/nfc/llcp_core.c b/net/nfc/llcp_core.c
index efe228f96..6baf2fc6b 100644
--- a/net/nfc/llcp_core.c
+++ b/net/nfc/llcp_core.c
@@ -1237,6 +1237,11 @@ static void nfc_llcp_recv_dm(struct nfc_llcp_local *local,
 	struct sock *sk;
 	u8 dsap, ssap, reason;
 
+	if (skb->len < LLCP_HEADER_SIZE + 1) {
+		pr_err("Malformed DM PDU\n");
+		return;
+	}
+
 	dsap = nfc_llcp_dsap(skb);
 	ssap = nfc_llcp_ssap(skb);
 	reason = skb->data[2];
-- 
2.51.0