[PATCH 3.16 108/370] block_dev: don't test bdev->bd_contains when it is not stable

Ben Hutchings
2017-03-10T14:06:24Z

From: NeilBrown <neilb@suse.com>

commit bcc7f5b4bee8e327689a4d994022765855c807ff upstream.

bdev->bd_contains is not stable before calling __blkdev_get().
When __blkdev_get() is called on a parition with ->bd_openers == 0
it sets
bdev->bd_contains = bdev;
which is not correct for a partition.
After a call to __blkdev_get() succeeds, ->bd_openers will be > 0
and then ->bd_contains is stable.

When FMODE_EXCL is used, blkdev_get() calls
bd_start_claiming() -> bd_prepare_to_claim() -> bd_may_claim()

This call happens before __blkdev_get() is called, so ->bd_contains
is not stable. So bd_may_claim() cannot safely use ->bd_contains.
It currently tries to use it, and this can lead to a BUG_ON().

This happens when a whole device is already open with a bd_holder (in
use by dm in my particular example) and two threads race to open a
partition of that device for the first time, one opening with O_EXCL and
one without.

The thread that doesn't use O_EXCL gets through blkdev_get() to
__blkdev_get(), gains the ->bd_mutex, and sets bdev->bd_contains = bdev;

Immediately thereafter the other thread, using FMODE_EXCL, calls
bd_start_claiming() from blkdev_get(). This should fail because the
whole device has a holder, but because bdev->bd_contains == bdev
bd_may_claim() incorrectly reports success.
This thread continues and blocks on bd_mutex.

The first thread then sets bdev->bd_contains correctly and drops the mutex.
The thread using FMODE_EXCL then continues and when it calls bd_may_claim()
again in:
BUG_ON(!bd_may_claim(bdev, whole, holder));
The BUG_ON fires.

Fix this by removing the dependency on ->bd_contains in
bd_may_claim(). As bd_may_claim() has direct access to the whole
device, it can simply test if the target bdev is the whole device.

Fixes: 6b4517a7913a ("block: implement bd_claiming and claiming block")
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/block_dev.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -698,7 +698,7 @@ static bool bd_may_claim(struct block_de
return true; /* already a holder */
else if (bdev->bd_holder != NULL)
return false; /* held by someone else */
- else if (bdev->bd_contains == bdev)
+ else if (whole == bdev)
return true; /* is a whole device which isn't held */

else if (whole->bd_holder == bd_may_claim)