Re: [PATCH] um: Add winch to winch_handlers before registering winch IRQ

[Roberto Sassu <roberto.sassu@huaweicloud.com>]

On 3/7/2024 1:43 PM, Roberto Sassu wrote:
> On Thu, 2024-03-07 at 11:49 +0100, Roberto Sassu wrote:
>> From: Roberto Sassu <roberto.sassu@huawei.com>
>>
>> Registering a winch IRQ is racy, an interrupt may occur before the winch is
>> added to the winch_handlers list.
>>
>> If that happens, register_winch_irq() adds to that list a winch that is
>> scheduled to be (or has already been) freed, causing a panic later in
>> winch_cleanup().
>>
>> Avoid the race by adding the winch to the winch_handlers list before
>> registering the IRQ, and rolling back if um_request_irq() fails.
>>
>> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> 
> Fixes: 42a359e31a0e ("uml: SIGIO support cleanup")
> 
> I see that before that commit there was the same ordering (list_add()
> before um_request_irq()).
> 
> Failure from um_request_irq() should not result in executing
> winch_interrupt() which could call list_del() itself. Then, it should
> be fine to delete the winch in the error path.

Richard, did you have time to look at this?

Thanks

Roberto

> Roberto
> 
>> ---
>>   arch/um/drivers/line.c | 14 ++++++++------
>>   1 file changed, 8 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/um/drivers/line.c b/arch/um/drivers/line.c
>> index ffc5cb92fa36..d82bc3fdb86e 100644
>> --- a/arch/um/drivers/line.c
>> +++ b/arch/um/drivers/line.c
>> @@ -676,24 +676,26 @@ void register_winch_irq(int fd, int tty_fd, int pid, struct tty_port *port,
>>   		goto cleanup;
>>   	}
>>   
>> -	*winch = ((struct winch) { .list  	= LIST_HEAD_INIT(winch->list),
>> -				   .fd  	= fd,
>> +	*winch = ((struct winch) { .fd  	= fd,
>>   				   .tty_fd 	= tty_fd,
>>   				   .pid  	= pid,
>>   				   .port 	= port,
>>   				   .stack	= stack });
>>   
>> +	spin_lock(&winch_handler_lock);
>> +	list_add(&winch->list, &winch_handlers);
>> +	spin_unlock(&winch_handler_lock);
>> +
>>   	if (um_request_irq(WINCH_IRQ, fd, IRQ_READ, winch_interrupt,
>>   			   IRQF_SHARED, "winch", winch) < 0) {
>>   		printk(KERN_ERR "register_winch_irq - failed to register "
>>   		       "IRQ\n");
>> +		spin_lock(&winch_handler_lock);
>> +		list_del(&winch->list);
>> +		spin_unlock(&winch_handler_lock);
>>   		goto out_free;
>>   	}
>>   
>> -	spin_lock(&winch_handler_lock);
>> -	list_add(&winch->list, &winch_handlers);
>> -	spin_unlock(&winch_handler_lock);
>> -
>>   	return;
>>   
>>    out_free: