[PATCH net v3 3/4] nfc: llcp: fix TLV parsing OOB in nfc_llcp_recv_snl

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

From: Lekë Hapçiu <framemain@outlook.com>

nfc_llcp_recv_snl() has four problems when handling a hostile peer:

 1. nfc_llcp_dsap()/nfc_llcp_ssap() dereference skb->data[0..1] without
    verifying skb->len; a 0- or 1-byte frame leads to an OOB read.
    Additionally tlv_len = skb->len - LLCP_HEADER_SIZE wraps when
    skb->len < 2, causing the following loop to run far past the
    buffer.

 2. The per-iteration loop guard `offset < tlv_len` only proves one
    byte is available, but the body reads tlv[0] and tlv[1].

 3. The peer-supplied `length` field is used to advance `tlv` without
    being checked against the remaining array space.

 4. The SDREQ handler previously only required length >= 1 but reads
    both tid (tlv[2]) and the first byte of service_name (tlv[3], via
    the pr_debug("%.16s") print and the service_name_len = length - 1
    string usage), so length >= 2 is required.

Fix: reject frames smaller than LLCP_HEADER_SIZE up front; add TLV
header and TLV value guards at the top of each iteration; bump the
SDREQ minimum length to 2.

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

Fixes: 7a06f0ee2823 ("NFC: llcp: Service Name Lookup implementation")
Cc: stable@vger.kernel.org
Signed-off-by: Lekë Hapçiu <framemain@outlook.com>
---
 net/nfc/llcp_core.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/net/nfc/llcp_core.c b/net/nfc/llcp_core.c
index 366d75663..efe228f96 100644
--- a/net/nfc/llcp_core.c
+++ b/net/nfc/llcp_core.c
@@ -1282,6 +1282,11 @@ static void nfc_llcp_recv_snl(struct nfc_llcp_local *local,
 	size_t sdres_tlvs_len;
 	HLIST_HEAD(nl_sdres_list);
 
+	if (skb->len < LLCP_HEADER_SIZE) {
+		pr_err("Malformed SNL PDU\n");
+		return;
+	}
+
 	dsap = nfc_llcp_dsap(skb);
 	ssap = nfc_llcp_ssap(skb);
 
@@ -1298,11 +1303,17 @@ static void nfc_llcp_recv_snl(struct nfc_llcp_local *local,
 	sdres_tlvs_len = 0;
 
 	while (offset < tlv_len) {
+		if (tlv_len - offset < 2)
+			break;
 		type = tlv[0];
 		length = tlv[1];
+		if (tlv_len - offset - 2 < length)
+			break;
 
 		switch (type) {
 		case LLCP_TLV_SDREQ:
+			if (length < 2)
+				break;
 			tid = tlv[2];
 			service_name = (char *) &tlv[3];
 			service_name_len = length - 1;
-- 
2.51.0