[PATCH 3.2 051/221] dm: fix a race condition in dm_get_md

Ben Hutchings
2015-05-05T01:54:27Z

From: Mikulas Patocka <mpatocka@redhat.com>

commit 2bec1f4a8832e74ebbe859f176d8a9cb20dd97f4 upstream.

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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/md/dm.c | 27 ++++++++++-----------------
1 file changed, 10 insertions(+), 17 deletions(-)

--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -2232,7 +2232,7 @@ int dm_setup_md_queue(struct mapped_devi
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);
@@ -2243,12 +2243,15 @@ static struct mapped_device *dm_find_md(
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:
@@ -2256,16 +2259,6 @@ 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)