nfc: s3fwrn5: allocate rx skb before consuming bytes

s3fwrn82_uart_read() reports the number of accepted bytes to the serdev
core. The current code consumes bytes into recv_skb and may already
deliver a complete frame before allocating a fresh receive buffer.

If that alloc_skb() fails, the callback returns 0 even though it has
already consumed bytes, and it leaves recv_skb as NULL for the next
receive callback. That breaks the receive_buf() accounting contract and
can also lead to a NULL dereference on the next skb_put_u8().

Allocate the receive skb lazily before consuming the next byte instead.
If allocation fails, return the number of bytes already accepted.

Fixes: 3f52c2c ("nfc: s3fwrn5: Support a UART interface")
Signed-off-by: Pengpeng Hou <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: neosys007

drivers/nfc/s3fwrn5/uart.c

@@ -58,6 +58,12 @@ static size_t s3fwrn82_uart_read(struct serdev_device *serdev,
 	size_t i;
 
 	for (i = 0; i < count; i++) {
+		if (!phy->recv_skb) {
+			phy->recv_skb = alloc_skb(NCI_SKB_BUFF_LEN, GFP_KERNEL);
+			if (!phy->recv_skb)
+				return i;
+		}
+
 		skb_put_u8(phy->recv_skb, *data++);
 
 		if (phy->recv_skb->len < S3FWRN82_NCI_HEADER)
@@ -69,9 +75,7 @@ static size_t s3fwrn82_uart_read(struct serdev_device *serdev,
 
 		s3fwrn5_recv_frame(phy->common.ndev, phy->recv_skb,
 				   phy->common.mode);
-		phy->recv_skb = alloc_skb(NCI_SKB_BUFF_LEN, GFP_KERNEL);
-		if (!phy->recv_skb)
-			return 0;
+		phy->recv_skb = NULL;
 	}
 
 	return i;