When the pn532 uart device is detaching, the pn532_uart_remove()
is called. But there are no functions in pn532_uart_remove() that
could delete the cmd_timeout timer, which will cause use-after-free
bugs. The process is shown below:

    (thread 1)                  |        (thread 2)
                                |  pn532_uart_send_frame
pn532_uart_remove               |    mod_timer(&pn532->cmd_timeout,...)
  ...                           |    (wait a time)
  kfree(pn532) //FREE           |    pn532_cmd_timeout
                                |      pn532_uart_send_frame
                                |        pn532->... //USE

This patch adds del_timer_sync() in pn532_uart_remove() in order to
prevent the use-after-free bugs. What's more, the pn53x_unregister_nfc()
is well synchronized, it sets nfc_dev->shutting_down to true and there
are no syscalls could restart the cmd_timeout timer.

Fixes: c656aa4c27b1 ("nfc: pn533: add UART phy driver")
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
---
 drivers/nfc/pn533/uart.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/nfc/pn533/uart.c b/drivers/nfc/pn533/uart.c
index 2caf997f9bc..07596bf5f7d 100644
--- a/drivers/nfc/pn533/uart.c
+++ b/drivers/nfc/pn533/uart.c
@@ -310,6 +310,7 @@ static void pn532_uart_remove(struct serdev_device *serdev)
 	pn53x_unregister_nfc(pn532->priv);
 	serdev_device_close(serdev);
 	pn53x_common_clean(pn532->priv);
+	del_timer_sync(&pn532->cmd_timeout);
 	kfree_skb(pn532->recv_skb);
 	kfree(pn532);
 }
-- 
2.17.1

Hello:

This patch was applied to netdev/net.git (master)
by David S. Miller <davem@davemloft.net>:

On Thu, 18 Aug 2022 17:06:21 +0800 you wrote:
> When the pn532 uart device is detaching, the pn532_uart_remove()
> is called. But there are no functions in pn532_uart_remove() that
> could delete the cmd_timeout timer, which will cause use-after-free
> bugs. The process is shown below:
> 
>     (thread 1)                  |        (thread 2)
>                                 |  pn532_uart_send_frame
> pn532_uart_remove               |    mod_timer(&pn532->cmd_timeout,...)
>   ...                           |    (wait a time)
>   kfree(pn532) //FREE           |    pn532_cmd_timeout
>                                 |      pn532_uart_send_frame
>                                 |        pn532->... //USE
> 
> [...]

Here is the summary with links:
  - [net] nfc: pn533: Fix use-after-free bugs caused by pn532_cmd_timeout
    https://git.kernel.org/netdev/net/c/f1e941dbf80a

You are awesome, thank you!
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