[PATCH net v5 3/3] nfc: llcp: fix TLV parsing OOB in nfc_llcp_connect_sn

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

nfc_llcp_connect_sn() walks the TLV array of an LLCP CONNECT PDU
looking for the Service Name TLV, but shares the same class of bugs
as nfc_llcp_recv_snl() / nfc_llcp_parse_gb_tlv():

 1. tlv_array_len = skb->len - LLCP_HEADER_SIZE wraps when skb->len
    is 0 or 1.  The subsequent loop then runs far past the buffer.

 2. The per-iteration guard `offset < tlv_array_len` only proves one
    byte is available, but the body reads both tlv[0] (type) and
    tlv[1] (length).

 3. The peer-supplied `length` field is used to advance `tlv` without
    being checked against the remaining array space, so a crafted
    length walks `tlv` past the buffer.  On the following iteration
    tlv[0]/tlv[1] are read from adjacent memory.

 4. When an LLCP_TLV_SN is found, the function returns &tlv[2] with
    *sn_len = length but without verifying that `length` bytes at
    tlv[2..] are still inside the TLV array.  The caller in
    nfc_llcp_recv_connect() then uses this (pointer, length) pair as
    a service name, so it may read past the PDU.

Fix: reject frames smaller than LLCP_HEADER_SIZE up front; add TLV
header and TLV value guards at the top of each iteration.  The value
guard also ensures that the (&tlv[2], length) pair returned on
LLCP_TLV_SN lies fully inside the TLV array.

Also use LLCP_HEADER_SIZE instead of the magic literal `2` to match
the style of neighbouring LLCP receive paths.

Reported-by: Simon Horman <horms@kernel.org>
Closes: https://lore.kernel.org/netdev/20260417160438.GH31784@horms.kernel.org/
Fixes: d646960f7986 ("NFC: Initial LLCP support")
Cc: stable@vger.kernel.org
Signed-off-by: Lekë Hapçiu <snowwlake@icloud.com>
---
 net/nfc/llcp_core.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/net/nfc/llcp_core.c b/net/nfc/llcp_core.c
index edec2fd83f79..c4dda8e7cfcf 100644
--- a/net/nfc/llcp_core.c
+++ b/net/nfc/llcp_core.c
@@ -849,12 +849,22 @@ static struct nfc_llcp_sock *nfc_llcp_sock_get_sn(struct nfc_llcp_local *local,
 static const u8 *nfc_llcp_connect_sn(const struct sk_buff *skb, size_t *sn_len)
 {
 	u8 type, length;
-	const u8 *tlv = &skb->data[2];
-	size_t tlv_array_len = skb->len - LLCP_HEADER_SIZE, offset = 0;
+	const u8 *tlv;
+	size_t tlv_array_len, offset = 0;
+
+	if (skb->len < LLCP_HEADER_SIZE)
+		return NULL;
+
+	tlv = &skb->data[LLCP_HEADER_SIZE];
+	tlv_array_len = skb->len - LLCP_HEADER_SIZE;
 
 	while (offset < tlv_array_len) {
+		if (tlv_array_len - offset < 2)
+			break;
 		type = tlv[0];
 		length = tlv[1];
+		if (tlv_array_len - offset - 2 < length)
+			break;
 
 		pr_debug("type 0x%x length %d\n", type, length);
 
-- 
2.51.0