[PATCH net] nfc: st21nfca: validate ATR_REQ length against the received frame

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

st21nfca_tm_recv_atr_req() checks that the received ATR_REQ frame is at
least ST21NFCA_ATR_REQ_MIN_SIZE and that the self-declared atr_req->length
is at least sizeof(struct st21nfca_atr_req), but never checks that
atr_req->length does not exceed the actual received length (skb->len).

st21nfca_tm_send_atr_res() then trusts the declared length:

	gb_len = atr_req->length - sizeof(struct st21nfca_atr_req);
	...
	memcpy(atr_res->gbi, atr_req->gbi, gb_len);

so an RF peer that sends a short frame but sets atr_req->length larger
than the frame makes gb_len exceed the general bytes actually present,
and the memcpy reads out of bounds past the received skb. Those bytes are
placed in the ATR_RES and sent back to the peer (kernel-memory disclosure
to a proximity attacker); a larger declared length is an out-of-bounds
read (DoS).

Reject frames whose declared length exceeds the received length. The
adjacent nfc_tm_activated() path in the same function already derives its
general-bytes length from skb->len rather than the declared field.

Found by 0sec (https://0sec.ai) using automated source analysis; the
missing bound is evident from source. Compile-tested.

Fixes: 1892bf844ea0 ("NFC: st21nfca: Adding P2P support to st21nfca in Initiator & Target mode")
Cc: stable@vger.kernel.org
Assisted-by: 0sec:claude-opus-4-8
Signed-off-by: Doruk Tan Ozturk <doruk@0sec.ai>
---
 drivers/nfc/st21nfca/dep.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/nfc/st21nfca/dep.c b/drivers/nfc/st21nfca/dep.c
index 3425b68f0ddc..a5fab4fd5129 100644
--- a/drivers/nfc/st21nfca/dep.c
+++ b/drivers/nfc/st21nfca/dep.c
@@ -205,6 +205,9 @@ static int st21nfca_tm_recv_atr_req(struct nfc_hci_dev *hdev,
 	if (atr_req->length < sizeof(struct st21nfca_atr_req))
 		return -EPROTO;
 
+	if (atr_req->length > skb->len)
+		return -EPROTO;
+
 	r = st21nfca_tm_send_atr_res(hdev, atr_req);
 	if (r)
 		return r;
-- 
2.43.0