nfcmrvl_nci_uart_open() stores the per-ldisc back-pointer nu->drv_data
only after nfcmrvl_nci_register_dev() has returned.  That function
registers the nci device, and a transmit started during registration
reaches the UART write path

  nci_uart_write_work() -> nu->ops.tx_start()
  == nfcmrvl_nci_uart_tx_start()

which dereferences nu->drv_data:

  struct nfcmrvl_private *priv = (struct nfcmrvl_private *)nu->drv_data;

  if (priv->ndev->nfc_dev->fw_download_in_progress)

At that point nu->drv_data is still NULL:

  BUG: KASAN: null-ptr-deref in nfcmrvl_nci_uart_tx_start+0x2c/0xc0
  Workqueue: events nci_uart_write_work
  Call Trace:
   nfcmrvl_nci_uart_tx_start
   nci_uart_write_work
   worker_thread

Publish nu->drv_data (and nu->ndev) for the UART phy before
nci_register_device() is called, and clear them again if registration
fails, so that a transmit triggered during registration finds a valid
pointer.  The now-redundant assignment in nfcmrvl_nci_uart_open() is
removed.

Fixes: e097dc624f78 ("NFC: nfcmrvl: add UART driver")
Assisted-by: LLM Claude
Signed-off-by: Shubham Antil <shubham@octane.security>
---
 drivers/nfc/nfcmrvl/main.c | 17 +++++++++++++++++
 drivers/nfc/nfcmrvl/uart.c |  3 ---
 2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/drivers/nfc/nfcmrvl/main.c b/drivers/nfc/nfcmrvl/main.c
index 6efa83219..8186e86d7 100644
--- a/drivers/nfc/nfcmrvl/main.c
+++ b/drivers/nfc/nfcmrvl/main.c
@@ -96,6 +96,7 @@ struct nfcmrvl_private *nfcmrvl_nci_register_dev(enum nfcmrvl_phy phy,
 				const struct nfcmrvl_platform_data *pdata)
 {
 	struct nfcmrvl_private *priv;
+	struct nci_uart *nu = NULL;
 	int rc;
 	int headroom;
 	int tailroom;
@@ -110,6 +111,9 @@ struct nfcmrvl_private *nfcmrvl_nci_register_dev(enum nfcmrvl_phy phy,
 	priv->dev = dev;
 	priv->phy = phy;
 
+	if (phy == NFCMRVL_PHY_UART)
+		nu = drv_data;
+
 	memcpy(&priv->config, pdata, sizeof(*pdata));
 
 	if (!priv->config.reset_gpio) {
@@ -154,9 +158,22 @@ struct nfcmrvl_private *nfcmrvl_nci_register_dev(enum nfcmrvl_phy phy,
 
 	nci_set_drvdata(priv->ndev, priv);
 
+	/* For the UART phy the transmit path reaches the driver through
+	 * nu->drv_data; publish it before nci_register_device() so a transmit
+	 * triggered during registration does not dereference a NULL pointer.
+	 */
+	if (nu) {
+		nu->drv_data = priv;
+		nu->ndev = priv->ndev;
+	}
+
 	rc = nci_register_device(priv->ndev);
 	if (rc) {
 		nfc_err(dev, "nci_register_device failed %d\n", rc);
+		if (nu) {
+			nu->drv_data = NULL;
+			nu->ndev = NULL;
+		}
 		goto error_fw_dnld_deinit;
 	}
 
diff --git a/drivers/nfc/nfcmrvl/uart.c b/drivers/nfc/nfcmrvl/uart.c
index 9aedd1687..421e6dc99 100644
--- a/drivers/nfc/nfcmrvl/uart.c
+++ b/drivers/nfc/nfcmrvl/uart.c
@@ -138,9 +138,6 @@ static int nfcmrvl_nci_uart_open(struct nci_uart *nu)
 
 	priv->support_fw_dnld = true;
 
-	nu->drv_data = priv;
-	nu->ndev = priv->ndev;
-
 	return 0;
 }
 
-- 
2.43.0