uio: Fix an Oops on load

BugLink: https://bugs.launchpad.net/bugs/1810958

commit 43279819 upstream.

I was trying to solve a double free but I introduced a more serious
NULL dereference bug.  The problem is that if there is an IRQ which
triggers immediately, then we need "info->uio_dev" but it's not set yet.

This patch puts the original initialization back to how it was and just
sets info->uio_dev to NULL on the error path so it should solve both
the Oops and the double free.

Fixes: f019f07e ("uio: potential double frees if __uio_register_device() fails")
Reported-by: Mathias Thore <Mathias.Thore@infinera.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: stable <stable@vger.kernel.org>
Tested-by: Mathias Thore <Mathias.Thore@infinera.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/uio/uio.c

@@ -842,6 +842,8 @@ int __uio_register_device(struct module *owner,
 	if (ret)
 		goto err_uio_dev_add_attributes;
 
+	info->uio_dev = idev;
+
 	if (info->irq && (info->irq != UIO_IRQ_CUSTOM)) {
 		/*
 		 * Note that we deliberately don't use devm_request_irq
@@ -853,11 +855,12 @@ int __uio_register_device(struct module *owner,
 		 */
 		ret = request_irq(info->irq, uio_interrupt,
 				  info->irq_flags, info->name, idev);
-		if (ret)
+		if (ret) {
+			info->uio_dev = NULL;
 			goto err_request_irq;
+		}
 	}
 
-	info->uio_dev = idev;
 	return 0;
 
 err_request_irq: