dm: fix a race condition in dm_get_md

The function dm_get_md finds a device mapper device with a given dev_t,
increases the reference count and returns the pointer.

dm_get_md calls dm_find_md, dm_find_md takes _minor_lock, finds the
device, tests that the device doesn't have DMF_DELETING or DMF_FREEING
flag, drops _minor_lock and returns pointer to the device. dm_get_md then
calls dm_get. dm_get calls BUG if the device has the DMF_FREEING flag,
otherwise it increments the reference count.

There is a possible race condition - after dm_find_md exits and before
dm_get is called, there are no locks held, so the device may disappear or
DMF_FREEING flag may be set, which results in BUG.

To fix this bug, we need to call dm_get while we hold _minor_lock. This
patch renames dm_find_md to dm_get_md and changes it so that it calls
dm_get while holding the lock.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org

1 files changed, 10 insertions, 17 deletions

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index ec1444f49de1..73f28802dc7a 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -2571,7 +2571,7 @@ int dm_setup_md_queue(struct mapped_device *md)
         return 0;
 }
 
-static struct mapped_device *dm_find_md(dev_t dev)
+struct mapped_device *dm_get_md(dev_t dev)
 {
         struct mapped_device *md;
         unsigned minor = MINOR(dev);
@@ -2582,12 +2582,15 @@ static struct mapped_device *dm_find_md(dev_t dev)
         spin_lock(&_minor_lock);
 
         md = idr_find(&_minor_idr, minor);
-        if (md && (md == MINOR_ALLOCED ||
-                   (MINOR(disk_devt(dm_disk(md))) != minor) ||
-                   dm_deleting_md(md) ||
-                   test_bit(DMF_FREEING, &md->flags))) {
-                md = NULL;
-                goto out;
+        if (md) {
+                if ((md == MINOR_ALLOCED ||
+                     (MINOR(disk_devt(dm_disk(md))) != minor) ||
+                     dm_deleting_md(md) ||
+                     test_bit(DMF_FREEING, &md->flags))) {
+                        md = NULL;
+                        goto out;
+                }
+                dm_get(md);
         }
 
 out:
@@ -2595,16 +2598,6 @@ out:
 
         return md;
 }
-
-struct mapped_device *dm_get_md(dev_t dev)
-{
-        struct mapped_device *md = dm_find_md(dev);
-
-        if (md)
-                dm_get(md);
-
-        return md;
-}
 EXPORT_SYMBOL_GPL(dm_get_md);
 
 void *dm_get_mdptr(struct mapped_device *md)