Re:Re: [PATCH] usb: gadget: serial: Fixed KASAN null-ptr-deref in tty_wakeup

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From: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Date: 2020-03-18 18:12:40
To:  Li Tao <tao.li@xxxxxxxx>
Cc:  Felipe Balbi <balbi@xxxxxxxxxx>,"MichaÅ MirosÅaw" <mirq-linux@xxxxxxxxxxxx>,Sergey Organov <sorganov@xxxxxxxxx>,linux-usb@xxxxxxxxxxxxxxx,linux-kernel@xxxxxxxxxxxxxxx,wenhu.wang@xxxxxxxx
Subject: Re: [PATCH] usb: gadget: serial: Fixed KASAN null-ptr-deref in tty_wakeup>On Wed, Mar 18, 2020 at 10:56:01AM +0800, Li Tao wrote:
>> The port->port.tty maybe reset as NULL, If gs_close() was invoked
>> unexpectedly during running gserial_connect().
>> 
>> BUG: KASAN: null-ptr-deref in tty_wakeup+0x1c/0x84
>> Call trace:
>> [<0000000095c3c837>] dump_backtrace+0x0/0x6d4
>> [<0000000047726bb8>] show_stack+0x18/0x20
>> [<00000000bedb4c1e>] --dump_stack+0x20/0x28
>> [<00000000722f2e2a>] dump_stack+0x84/0xb0
>> [<00000000325683d4>] kasan_report_error+0x178/0x1e4
>> [<0000000053079998>] kasan_report_error+0x0/0x1e4
>> [<00000000b6d33afa>] --asan_load8+0x150/0x15c
>> [<00000000188745b8>] tty_wakeup+0x1c/0x84
>> [<0000000064f6dd21>] gs_start_io+0xd0/0x11c
>> [<0000000063d67b6c>] gserial_connect+0x15c/0x1b0
>> [<00000000faf7c0f9>] dm_set_alt+0xa8/0x190
>> [<000000008deb1909>] composite_setup+0xde4/0x1edc
>> [<00000000792ee16d>] android_setup+0x210/0x294
>> [<00000000ab32ef30>] dwc3_ep0_delegate_req+0x48/0x68
>> [<0000000054e26fd2>] dwc3_ep0_interrupt+0xf2c/0x16fc
>> [<0000000050cb2262>] dwc3_interrupt+0x464/0x1f44
>> [<00000000fdcaa6e9>] --handle_irq_event_percpu+0x184/0x398
>> [<000000003b24ff56>] handle_irq_event_percpu+0xa0/0x134
>> [<00000000aedda5ee>] handle_irq_event+0x60/0xa0
>> [<000000005f51a570>] handle_fasteoi_irq+0x23c/0x31c
>> [<000000008db2608d>] generic_handle_irq+0x70/0xa4
>> [<00000000098683fc>] --handle_domain_irq+0x84/0xe0
>> [<000000008ed23b46>] gic_handle_irq+0x98/0xb8
>> 
>> Signed-off-by: Li Tao <tao.li@xxxxxxxx>
>> ---
>>  drivers/usb/gadget/function/u_serial.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>> 
>> diff --git a/drivers/usb/gadget/function/u_serial.c b/drivers/usb/gadget/function/u_serial.c
>> index 8167d379e115..3c109a8f9ec4 100644
>> --- a/drivers/usb/gadget/function/u_serial.c
>> +++ b/drivers/usb/gadget/function/u_serial.c
>> @@ -565,7 +565,8 @@ static int gs_start_io(struct gs_port *port)
>>  		gs_start_tx(port);
>>  		/* Unblock any pending writes into our circular buffer, in case
>>  		 * we didn't in gs_start_tx() */
>> -		tty_wakeup(port->port.tty);
>> +		if (port->port.tty)
>> +			tty_wakeup(port->port.tty);
>
>What prevents port.tty from being set to NULL between the check and the
>call to tty_wakeup?
>
>Shouldn't gs_close() and gserial_connect() have some sort of locking to
>prevent this?
>
I encountered this issue on Samsung EXYNOS980 platform, Maybe this is platform specific issue.
The USB interrupt handler got port_lock on CPU 0 in gserial_connect while gs_close is waiting for 
this lock on CPU 1.  But this port_lock will unlock in gs_start_io-->gs_start_rx temporarily, This race
condition may cause port.tty set as NULL.

greg k-h, Do you have some good suggestions?

>thanks,
>
>greg k-h