[PATCH net v3] nfc: digital: check DEP_REQ length before reading DID byte

digital_tg_recv_dep_req() reads resp->data[3] when the DID bit is set
in the PFB, but only sizeof(struct digital_dep_req_res) bytes (3) are
guaranteed by the preceding length check.  A crafted DEP_REQ that is
exactly 3 bytes long with the DID bit set causes an out-of-bounds read
past the valid skb data.

This read can happen on every DEP_REQ from a malicious NFC initiator
when the target device has a non-zero DID.  The stale byte is compared
against ddev->did, so in practice the mismatch causes an -EIO return,
but the read itself is undefined behavior and accesses data past the
buffer.

Reorder the condition so that the length check (resp->len < size + 1)
comes first and short-circuits before resp->data[size] is accessed.
The later "size > resp->len" check becomes redundant and is left in
place.

Fixes: 05afedcb8918 ("NFC: digital: Add Target-mode NFC-DEP DID Support")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Aamir Ahmed <redacted>
Reviewed-by: Simon Horman <horms@kernel.org>
---
v3:
  - correct the Fixes: tag; the bug came in with 05afedcb8918
  - reword the note on the later "size > resp->len" check, which
    returns early for NAD frames and is now redundant rather than
    preserved for that case
  No code change; Simon's Reviewed-by carried.
v2: https://lore.kernel.org/netdev/AS8P251MB0001F45C42BBD0D16D7B30ABC8BD2@AS8P251MB0001.EURP251.PROD.OUTLOOK.COM/ (local)
  - add the Assisted-by: LLM tag (Simon)
  - designate the target tree in the subject
  - collect Simon's Reviewed-by
v1: https://lore.kernel.org/netdev/AS8P251MB000196084524EB27D62162B8C8B32@AS8P251MB0001.EURP251.PROD.OUTLOOK.COM/ (local)

Compile-tested only; I have no NFC hardware.

 net/nfc/digital_dep.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/nfc/digital_dep.c b/net/nfc/digital_dep.c
index 3982fa084737..ae5407fca12d 100644
--- a/net/nfc/digital_dep.c
+++ b/net/nfc/digital_dep.c

@@ -1117,12 +1117,12 @@ static void digital_tg_recv_dep_req(struct nfc_digital_dev *ddev, void *arg,
 	pfb = dep_req->pfb;

 	if (DIGITAL_NFC_DEP_DID_BIT_SET(pfb)) {
-		if (ddev->did && (ddev->did == resp->data[3])) {
-			size++;
-		} else {
+		if (resp->len < size + 1 || !ddev->did ||
+		    ddev->did != resp->data[size]) {
 			rc = -EIO;
 			goto exit;
 		}
+		size++;
 	} else if (ddev->did) {
 		rc = -EIO;
 		goto exit;

base-commit: df2908090cda368b01ff43709f51890076c56157
-- 
2.55.0