[PATCH 4.4 030/101] block: fix module reference leak on put_disk() call for cgroups throttle

Greg Kroah-Hartman
2017-07-03T13:34:30Z

From: Roman Pen <roman.penyaev@profitbricks.com>

commit 39a169b62b415390398291080dafe63aec751e0a upstream.

get_disk(),get_gendisk() calls have non explicit side effect: they
increase the reference on the disk owner module.

The following is the correct sequence how to get a disk reference and
to put it:

disk = get_gendisk(...);

/* use disk */

owner = disk->fops->owner;
put_disk(disk);
module_put(owner);

fs/block_dev.c is aware of this required module_put() call, but f.e.
blkg_conf_finish(), which is located in block/blk-cgroup.c, does not put
a module reference. To see a leakage in action cgroups throttle config
can be used. In the following script I'm removing throttle for /dev/ram0
(actually this is NOP, because throttle was never set for this device):

# lsmod | grep brd
brd 5175 0
# i=100; while [ $i -gt 0 ]; do echo "1:0 0" > \
/sys/fs/cgroup/blkio/blkio.throttle.read_bps_device; i=$(($i - 1)); \
done
# lsmod | grep brd
brd 5175 100

Now brd module has 100 references.

The issue is fixed by calling module_put() just right away put_disk().

Signed-off-by: Roman Pen <roman.penyaev@profitbricks.com>
Cc: Gi-Oh Kim <gi-oh.kim@profitbricks.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@fb.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
block/blk-cgroup.c | 9 +++++++++
1 file changed, 9 insertions(+)

--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -788,6 +788,7 @@ int blkg_conf_prep(struct blkcg *blkcg,
{
struct gendisk *disk;
struct blkcg_gq *blkg;
+ struct module *owner;
unsigned int major, minor;
int key_len, part, ret;
char *body;
@@ -804,7 +805,9 @@ int blkg_conf_prep(struct blkcg *blkcg,
if (!disk)
return -ENODEV;
if (part) {
+ owner = disk->fops->owner;
put_disk(disk);
+ module_put(owner);
return -ENODEV;
}

@@ -820,7 +823,9 @@ int blkg_conf_prep(struct blkcg *blkcg,
ret = PTR_ERR(blkg);
rcu_read_unlock();
spin_unlock_irq(disk->queue->queue_lock);
+ owner = disk->fops->owner;
put_disk(disk);
+ module_put(owner);
/*
* If queue was bypassing, we should retry. Do so after a
* short msleep(). It isn't strictly necessary but queue
@@ -851,9 +856,13 @@ EXPORT_SYMBOL_GPL(blkg_conf_prep);
void blkg_conf_finish(struct blkg_conf_ctx *ctx)
__releases(ctx->disk->queue->queue_lock) __releases(rcu)
{
+ struct module *owner;
+
spin_unlock_irq(ctx->disk->queue->queue_lock);
rcu_read_unlock();
+ owner = ctx->disk->fops->owner;
put_disk(ctx->disk);
+ module_put(owner);
}
EXPORT_SYMBOL_GPL(blkg_conf_finish);