Re: [PATCH 1/2] serdev: ttyport: fix use-after-free on closed TTY

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Dec 21, 2022 at 05:32:48PM +0100, Krzysztof Kozlowski wrote:
> use-after-free is visible in serdev-ttyport, e.g. during system reboot
> with Qualcomm Atheros Bluetooth.  The TTY is closed, thus "struct
> tty_struct" is being released, but the hci_uart_qca driver performs
> writes and flushes during system shutdown in qca_serdev_shutdown().
> 
>   Unable to handle kernel paging request at virtual address 0072662f67726fd7
>   ...
>   CPU: 6 PID: 1 Comm: systemd-shutdow Tainted: G        W          6.1.0-rt5-00325-g8a5f56bcfcca #8
>   Hardware name: Qualcomm Technologies, Inc. Robotics RB5 (DT)
>   Call trace:
>    tty_driver_flush_buffer+0x4/0x30
>    serdev_device_write_flush+0x24/0x34
>    qca_serdev_shutdown+0x80/0x130 [hci_uart]
>    device_shutdown+0x15c/0x260
>    kernel_restart+0x48/0xac
> 
> KASAN report:
> 
>   BUG: KASAN: use-after-free in tty_driver_flush_buffer+0x1c/0x50
>   Read of size 8 at addr ffff16270c2e0018 by task systemd-shutdow/1
> 
>   CPU: 7 PID: 1 Comm: systemd-shutdow Not tainted 6.1.0-next-20221220-00014-gb85aaf97fb01-dirty #28
>   Hardware name: Qualcomm Technologies, Inc. Robotics RB5 (DT)
>   Call trace:
>    dump_backtrace.part.0+0xdc/0xf0
>    show_stack+0x18/0x30
>    dump_stack_lvl+0x68/0x84
>    print_report+0x188/0x488
>    kasan_report+0xa4/0xf0
>    __asan_load8+0x80/0xac
>    tty_driver_flush_buffer+0x1c/0x50
>    ttyport_write_flush+0x34/0x44
>    serdev_device_write_flush+0x48/0x60
>    qca_serdev_shutdown+0x124/0x274
>    device_shutdown+0x1e8/0x350
>    kernel_restart+0x48/0xb0
>    __do_sys_reboot+0x244/0x2d0
>    __arm64_sys_reboot+0x54/0x70
>    invoke_syscall+0x60/0x190
>    el0_svc_common.constprop.0+0x7c/0x160
>    do_el0_svc+0x44/0xf0
>    el0_svc+0x2c/0x6c
>    el0t_64_sync_handler+0xbc/0x140
>    el0t_64_sync+0x190/0x194
> 
> Fixes: bed35c6dfa6a ("serdev: add a tty port controller driver")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
> ---
>  drivers/tty/serdev/serdev-ttyport.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/drivers/tty/serdev/serdev-ttyport.c b/drivers/tty/serdev/serdev-ttyport.c
> index d367803e2044..3d2bab91a988 100644
> --- a/drivers/tty/serdev/serdev-ttyport.c
> +++ b/drivers/tty/serdev/serdev-ttyport.c
> @@ -91,6 +91,9 @@ static void ttyport_write_flush(struct serdev_controller *ctrl)
>  	struct serport *serport = serdev_controller_get_drvdata(ctrl);
>  	struct tty_struct *tty = serport->tty;
>  
> +	if (!test_bit(SERPORT_ACTIVE, &serport->flags))
> +		return;

Shouldn't that be a more useful macro/function instead?
	serport_is_active(serport)

Anyway, what prevents this from changing _right_ after you test it and
before you call the next line in this function (same for all invocations
here.)

thanks,

greg k-h