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

Roberto Sassu
2024-03-07T12:43:54Z

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;
>