[PATCH net v4] nfc: llcp: bound SNL TLV parsing to the skb and add length checks

[Doruk Tan Ozturk <doruk@0sec.ai>]

nfc_llcp_recv_snl() walked the SNL TLV list using a u16 offset/length
pair derived from skb->len, without bounding reads to the actual skb
data. Three problems followed:

  - For a short frame (skb->len < LLCP_HEADER_SIZE), tlv_len underflowed.
  - The per-TLV header (type, length) was read without checking that two
    bytes remained.
  - A declared TLV length could run past the end of the buffer, and an
    SDREQ with length == 0 made "service_name_len = length - 1" underflow
    (size_t), driving an out-of-bounds read in the following strncmp() /
    nfc_llcp_sock_from_sn(). The SDRES case likewise read tlv[2]/tlv[3]
    without a length check.

A nearby NFC device can reach this without authentication; LLCP link
activation happens automatically after NFC-DEP.

Walk the TLV list by pointer, bounded by skb_tail_pointer() over the
linear skb data, and validate each TLV declared length before use. Add
explicit length checks for SDREQ (>= 1) and SDRES (exactly 2).

Found by 0sec automated security-research tooling (https://0sec.ai).

Fixes: 19cfe5843e86 ("NFC: Initial SNL support")
Signed-off-by: Doruk Tan Ozturk <doruk@0sec.ai>
---
v4:
 - Add Fixes: tag pointing at the commit that introduced the unbounded
   SNL TLV walk and the SDREQ service_name_len underflow (David Heidelberg).

v3 (review cleanups, no functional change to the fix):
 - Comment that only the linear part of the skb is parsed (David Laight).
 - Use int for service_name_len and print the bounded service name
   directly with %.*s; drop the min_t()/cast (David Laight).
 - Require SDRES length to be exactly 2, not just >= 2 (David Laight).

v2: https://lore.kernel.org/netdev/20260603135935.62647-1-doruk@0sec.ai/
 - Walk by pointer bounded on skb_tail_pointer(); drop the 16-bit
   offset/tlv_len math and fix the short-frame underflow (David Laight).
 - Add an SDRES length check alongside SDREQ length >= 1 (David Laight).
 - Bound the SDREQ service-name pr_debug to the field length.
 - Rebased onto linux-nfc for-next (David Heidelberg).

 net/nfc/llcp_core.c | 29 +++++++++++++++++++++--------
 1 file changed, 21 insertions(+), 8 deletions(-)

diff --git a/net/nfc/llcp_core.c b/net/nfc/llcp_core.c
index dc65c719f..aed5fe1af 100644
--- a/net/nfc/llcp_core.c
+++ b/net/nfc/llcp_core.c
@@ -1286,10 +1286,9 @@ static void nfc_llcp_recv_snl(struct nfc_llcp_local *local,
 {
 	struct nfc_llcp_sock *llcp_sock;
 	u8 dsap, ssap, type, length, tid, sap;
-	const u8 *tlv;
-	u16 tlv_len, offset;
+	const u8 *tlv, *tlv_end;
 	const char *service_name;
-	size_t service_name_len;
+	int service_name_len;
 	struct nfc_llcp_sdp_tlv *sdp;
 	HLIST_HEAD(llc_sdres_list);
 	size_t sdres_tlvs_len;
@@ -1305,22 +1304,34 @@ static void nfc_llcp_recv_snl(struct nfc_llcp_local *local,
 		return;
 	}
 
+	/*
+	 * Walk the SNL TLV list in the linear part of the skb only,
+	 * bounded by skb_tail_pointer(). Each TLV needs a two-byte
+	 * header (type, length) and its declared length must fit before
+	 * the end; this also keeps the walk safe for very short frames.
+	 */
 	tlv = &skb->data[LLCP_HEADER_SIZE];
-	tlv_len = skb->len - LLCP_HEADER_SIZE;
-	offset = 0;
+	tlv_end = skb_tail_pointer(skb);
 	sdres_tlvs_len = 0;
 
-	while (offset < tlv_len) {
+	while (tlv + 2 < tlv_end) {
 		type = tlv[0];
 		length = tlv[1];
 
+		if (tlv + 2 + length > tlv_end)
+			break;
+
 		switch (type) {
 		case LLCP_TLV_SDREQ:
+			if (length < 1)
+				break;
+
 			tid = tlv[2];
 			service_name = (char *) &tlv[3];
 			service_name_len = length - 1;
 
-			pr_debug("Looking for %.16s\n", service_name);
+			pr_debug("Looking for %.*s\n", service_name_len,
+				 service_name);
 
 			if (service_name_len == strlen("urn:nfc:sn:sdp") &&
 			    !strncmp(service_name, "urn:nfc:sn:sdp",
@@ -1380,6 +1391,9 @@ static void nfc_llcp_recv_snl(struct nfc_llcp_local *local,
 			break;
 
 		case LLCP_TLV_SDRES:
+			if (length != 2)
+				break;
+
 			mutex_lock(&local->sdreq_lock);
 
 			pr_debug("LLCP_TLV_SDRES: searching tid %d\n", tlv[2]);
@@ -1408,7 +1422,6 @@ static void nfc_llcp_recv_snl(struct nfc_llcp_local *local,
 			break;
 		}
 
-		offset += length + 2;
 		tlv += length + 2;
 	}
 
-- 
2.53.0