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

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

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.

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: