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

[Roberto Sassu]

From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>

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@xxxxxxxxxx>
---
 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:
-- 
2.34.1