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Subject[patch 043/100] dm table: fix upgrade mode race
-stable review patch.  If anyone has any objections, please let us know.
---------------------

From: Alasdair G Kergon <agk@redhat.com>

upstream commit: 570b9d968bf9b16974252ef7cbce73fa6dac34f3

upgrade_mode() sets bdev to NULL temporarily, and does not have any
locking to exclude anything from seeing that NULL.

In dm_table_any_congested() bdev_get_queue() can dereference that NULL and
cause a reported oops.

Fix this by not changing that field during the mode upgrade.

Cc: stable@kernel.org
Cc: Neil Brown <neilb@suse.de>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
---
drivers/md/dm-table.c | 26 ++++++++++++++------------
1 file changed, 14 insertions(+), 12 deletions(-)

--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -399,28 +399,30 @@ static int check_device_area(struct dm_d
}

/*
- * This upgrades the mode on an already open dm_dev. Being
+ * This upgrades the mode on an already open dm_dev, being
* careful to leave things as they were if we fail to reopen the
- * device.
+ * device and not to touch the existing bdev field in case
+ * it is accessed concurrently inside dm_table_any_congested().
*/
static int upgrade_mode(struct dm_dev_internal *dd, fmode_t new_mode,
struct mapped_device *md)
{
int r;
- struct dm_dev_internal dd_copy;
- dev_t dev = dd->dm_dev.bdev->bd_dev;
+ struct dm_dev_internal dd_new, dd_old;

- dd_copy = *dd;
+ dd_new = dd_old = *dd;
+
+ dd_new.dm_dev.mode |= new_mode;
+ dd_new.dm_dev.bdev = NULL;
+
+ r = open_dev(&dd_new, dd->dm_dev.bdev->bd_dev, md);
+ if (r)
+ return r;

dd->dm_dev.mode |= new_mode;
- dd->dm_dev.bdev = NULL;
- r = open_dev(dd, dev, md);
- if (!r)
- close_dev(&dd_copy, md);
- else
- *dd = dd_copy;
+ close_dev(&dd_old, md);

- return r;
+ return 0;
}

/*


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