[PATCH net] nfc: fdp: bound the device-supplied read size in fdp_nci_i2c_read()

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

fdp_nci_i2c_read() reads a "length packet" from the FDP I2C controller and
computes the size of the next I2C transfer from two device-supplied bytes:

	phy->next_read_size = (tmp[2] << 8) + tmp[3] + 3;

next_read_size is a u16 (up to 65535) and is never bounded. On the next
loop iteration it is used directly as the length passed to

	i2c_master_recv(client, tmp, len);

which reads into the fixed 261-byte stack buffer
tmp[FDP_NCI_I2C_MAX_PAYLOAD]. A malicious or malfunctioning controller
that reports a large length thus overflows the stack buffer -- the
r != len check runs only after the read has already happened.

Reject a next-read size larger than the buffer and resynchronize.

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

Fixes: a06347c04c13 ("NFC: Add Intel Fields Peak NFC solution driver")
Cc: stable@vger.kernel.org
Assisted-by: 0sec:claude-opus-4-8
Signed-off-by: Doruk Tan Ozturk <doruk@0sec.ai>
---
 drivers/nfc/fdp/i2c.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/nfc/fdp/i2c.c b/drivers/nfc/fdp/i2c.c
index c1896a1d978c..581f85f0dfa8 100644
--- a/drivers/nfc/fdp/i2c.c
+++ b/drivers/nfc/fdp/i2c.c
@@ -128,7 +128,7 @@ static const struct nfc_phy_ops i2c_phy_ops = {
 
 static int fdp_nci_i2c_read(struct fdp_i2c_phy *phy, struct sk_buff **skb)
 {
-	int r, len;
+	int r = -EREMOTEIO, len;
 	u8 tmp[FDP_NCI_I2C_MAX_PAYLOAD], lrc, k;
 	u16 i;
 	struct i2c_client *client = phy->i2c_dev;
@@ -140,6 +140,13 @@ static int fdp_nci_i2c_read(struct fdp_i2c_phy *phy, struct sk_buff **skb)
 
 		len = phy->next_read_size;
 
+		if (len > FDP_NCI_I2C_MAX_PAYLOAD) {
+			dev_dbg(&client->dev, "%s: read size %d too large\n",
+				__func__, len);
+			phy->next_read_size = FDP_NCI_I2C_MIN_PAYLOAD;
+			goto flush;
+		}
+
 		r = i2c_master_recv(client, tmp, len);
 		if (r != len) {
 			dev_dbg(&client->dev, "%s: i2c recv err: %d\n",
-- 
2.43.0