linux-kernel - [PATCH] loop: fix to a race condition due to the early registration of device

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Date:   Mon,  7 Aug 2017 15:37:50 +0300
From:   Anton Volkov <avolkov@...ras.ru>
To:     tom.leiming@...il.com
Cc:     axboe@...com, osandov@...com, linux-kernel@...r.kernel.org,
        linux-block@...r.kernel.org, ldv-project@...uxtesting.org,
        khoroshilov@...ras.ru, Anton Volkov <avolkov@...ras.ru>
Subject: [PATCH] loop: fix to a race condition due to the early registration of device

The early device registration made possible a race leading to allocations
of disks with wrong minors.

This patch moves the device registration further down the loop_init
function to make the race infeasible.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Anton Volkov <avolkov@...ras.ru>
Reviewed-by: Ming Lei <ming.lei@...hat.com>
---
 drivers/block/loop.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index ef83349..2fbd4089 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1996,10 +1996,6 @@ static int __init loop_init(void)
 	struct loop_device *lo;
 	int err;
 
-	err = misc_register(&loop_misc);
-	if (err < 0)
-		return err;
-
 	part_shift = 0;
 	if (max_part > 0) {
 		part_shift = fls(max_part);
@@ -2017,12 +2013,12 @@ static int __init loop_init(void)
 
 	if ((1UL << part_shift) > DISK_MAX_PARTS) {
 		err = -EINVAL;
-		goto misc_out;
+		goto err_out;
 	}
 
 	if (max_loop > 1UL << (MINORBITS - part_shift)) {
 		err = -EINVAL;
-		goto misc_out;
+		goto err_out;
 	}
 
 	/*
@@ -2041,6 +2037,11 @@ static int __init loop_init(void)
 		range = 1UL << MINORBITS;
 	}
 
+	err = misc_register(&loop_misc);
+	if (err < 0)
+		goto err_out;
+
+
 	if (register_blkdev(LOOP_MAJOR, "loop")) {
 		err = -EIO;
 		goto misc_out;
@@ -2060,6 +2061,7 @@ static int __init loop_init(void)
 
 misc_out:
 	misc_deregister(&loop_misc);
+err_out:
 	return err;
 }
 
-- 
2.7.4