Re: [PATCH] tty: serial: uartlite: avoid null pointer dereference during rmmod

[Johan Hovold]

On Tue, May 14, 2019 at 11:32:19AM +0800, Kefeng Wang wrote:
> After commit 415b43bdb008 "tty: serial: uartlite: Move uart register to
> probe", calling uart_unregister_driver unconditionally will trigger a
> null pointer dereference due to ulite_uart_driver may not registed.
> 
>   CPU: 1 PID: 3755 Comm: syz-executor.0 Not tainted 5.1.0+ #28
>   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
>   Call Trace:
>    __dump_stack lib/dump_stack.c:77 [inline]
>    dump_stack+0xa9/0x10e lib/dump_stack.c:113
>    __kasan_report+0x171/0x18d mm/kasan/report.c:321
>    kasan_report+0xe/0x20 mm/kasan/common.c:614
>    tty_unregister_driver+0x19/0x100 drivers/tty/tty_io.c:3383
>    uart_unregister_driver+0x30/0xc0 drivers/tty/serial/serial_core.c:2579
>    __do_sys_delete_module kernel/module.c:1027 [inline]
>    __se_sys_delete_module kernel/module.c:970 [inline]
>    __x64_sys_delete_module+0x244/0x330 kernel/module.c:970
>    do_syscall_64+0x72/0x2a0 arch/x86/entry/common.c:298
>    entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
> Call uart_unregister_driver only if ulite_uart_driver.state not null to
> fix it.
> 
> Cc: Peter Korsgaard <jacmet@xxxxxxxxxx>
> Cc: Shubhrajyoti Datta <shubhrajyoti.datta@xxxxxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Reported-by: Hulk Robot <hulkci@xxxxxxxxxx>
> Fixes: 415b43bdb008 ("tty: serial: uartlite: Move uart register to probe")
> Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
> ---
>  drivers/tty/serial/uartlite.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/uartlite.c b/drivers/tty/serial/uartlite.c
> index b8b912b5a8b9..06e79c11141d 100644
> --- a/drivers/tty/serial/uartlite.c
> +++ b/drivers/tty/serial/uartlite.c
> @@ -897,7 +897,8 @@ static int __init ulite_init(void)
>  static void __exit ulite_exit(void)
>  {
>  	platform_driver_unregister(&ulite_platform_driver);
> -	uart_unregister_driver(&ulite_uart_driver);
> +	if (ulite_uart_driver.state)
> +		uart_unregister_driver(&ulite_uart_driver);
>  }
>  
>  module_init(ulite_init);

This looks like you're just papering over the real issue, which is the
crazy idea of ultimately registering one driver per port:

	https://lkml.kernel.org/r/1539685088-13465-1-git-send-email-shubhrajyoti.datta@xxxxxxxxx

It appears only the preparatory patches from that series were applied,
and I think whoever is responsible should consider reverting those
instead.

If the statically allocated port state is that big of any issue, you
need to make serial core support dynamic allocation.

Johan