linux-kernel - [PATCH] loop: Fix use of unsafe lo->lo

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Date:   Thu, 25 Mar 2021 17:22:03 +0800
From:   qiang.zhang@...driver.com
To:     axboe@...nel.dk
Cc:     pasha.tatashin@...een.com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] loop: Fix use of unsafe lo->lo_mutex locks

From: Zqiang <qiang.zhang@...driver.com>

lo->lo_refcnt = 0

	    CPU0                                 CPU1
lo_open()                                    lo_open()
 mutex_lock(&lo->lo_mutex)                 
 atomic_inc(&lo->lo_refcnt)              
 lo_refcnt == 1
 mutex_unlock(&lo->lo_mutex)                     
                                             mutex_lock(&lo->lo_mutex)
                                             atomic_inc(&lo->lo_refcnt)
                                             lo_refcnt == 2
                                             mutex_unlock(&lo->lo_mutex)
loop_clr_fd()
 mutex_lock(&lo->lo_mutex)
 atomic_read(&lo->lo_refcnt) > 1
 lo->lo_flags |= LO_FLAGS_AUTOCLEAR        lo_release()
 mutex_unlock(&lo->lo_mutex)                    
 return                                      mutex_lock(&lo->lo_mutex)
 					   atomic_dec_return(&lo->lo_refcnt)
                                             lo_refcnt == 1
                                             mutex_unlock(&lo->lo_mutex)
                                             return

lo_release()
 mutex_lock(&lo->lo_mutex)
 atomic_dec_return(&lo->lo_refcnt)
 lo_refcnt == 0
 lo->lo_flags & LO_FLAGS_AUTOCLEAR 
  == true
 mutex_unlock(&lo->lo_mutex)              loop_control_ioctl()
                                           case LOOP_CTL_REMOVE:
                                            mutex_lock(&lo->lo_mutex)
                                            atomic_read(&lo->lo_refcnt)==0
  __loop_clr_fd(lo, true)                   mutex_unlock(&lo->lo_mutex)
    mutex_lock(&lo->lo_mutex)                loop_remove(lo)
                                               mutex_destroy(&lo->lo_mutex)
  ......                                       kfree(lo)
       UAF

When different tasks on two CPUs perform the above operations on the same
lo device, UAF may occur.

Fixes: 6cc8e7430801 ("loop: scale loop device by introducing per device lock")
Signed-off-by: Zqiang <qiang.zhang@...driver.com>
---
 drivers/block/loop.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index d58d68f3c7cd..5712f1698a66 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1201,7 +1201,6 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
 	bool partscan = false;
 	int lo_number;
 
-	mutex_lock(&lo->lo_mutex);
 	if (WARN_ON_ONCE(lo->lo_state != Lo_rundown)) {
 		err = -ENXIO;
 		goto out_unlock;
@@ -1257,7 +1256,6 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
 	lo_number = lo->lo_number;
 	loop_unprepare_queue(lo);
 out_unlock:
-	mutex_unlock(&lo->lo_mutex);
 	if (partscan) {
 		/*
 		 * bd_mutex has been held already in release path, so don't
@@ -1288,12 +1286,11 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
 	 * protects us from all the other places trying to change the 'lo'
 	 * device.
 	 */
-	mutex_lock(&lo->lo_mutex);
+
 	lo->lo_flags = 0;
 	if (!part_shift)
 		lo->lo_disk->flags |= GENHD_FL_NO_PART_SCAN;
 	lo->lo_state = Lo_unbound;
-	mutex_unlock(&lo->lo_mutex);
 
 	/*
 	 * Need not hold lo_mutex to fput backing file. Calling fput holding
@@ -1332,9 +1329,10 @@ static int loop_clr_fd(struct loop_device *lo)
 		return 0;
 	}
 	lo->lo_state = Lo_rundown;
+	err = __loop_clr_fd(lo, false);
 	mutex_unlock(&lo->lo_mutex);
 
-	return __loop_clr_fd(lo, false);
+	return err;
 }
 
 static int
@@ -1916,13 +1914,12 @@ static void lo_release(struct gendisk *disk, fmode_t mode)
 		if (lo->lo_state != Lo_bound)
 			goto out_unlock;
 		lo->lo_state = Lo_rundown;
-		mutex_unlock(&lo->lo_mutex);
 		/*
 		 * In autoclear mode, stop the loop thread
 		 * and remove configuration after last close.
 		 */
 		__loop_clr_fd(lo, true);
-		return;
+		goto out_unlock;
 	} else if (lo->lo_state == Lo_bound) {
 		/*
 		 * Otherwise keep thread (if running) and config,
-- 
2.17.1