[PATCH] ocfs2/heartbeat: serialize concurrent writes to a region's dev attribute

[Eklavya Singh <eklavyasingh2106@gmail.com>]

o2hb_region_dev_store() checks reg->hr_bdev_file to see if the region
is already started, but nothing stops two writers from doing that at
the same time. configfs only serializes writes through the same open
file, so two processes that each open the dev attribute can both get
past the check:

  A: hr_bdev_file is NULL, opens file A, sets hr_bdev_file = A
  B: hr_bdev_file is NULL, opens file B, sets hr_bdev_file = B
  A: start fails, fput(hr_bdev_file) drops B, sets it to NULL
  B: start fails, fput(NULL)

Both calls also use the same hr_slots, hr_slot_data and hr_tmp_block,
so the error paths can free the slot data twice.

syzbot hit the fput(NULL) with an unbound loop device, where every
start fails with -EIO. With four threads doing the same thing on a
KASAN kernel I get both crashes within a second:

  BUG: KASAN: double-free in o2hb_unmap_slot_data+0x14d/0x2f0
  BUG: kernel NULL pointer dereference, address: 0000000000000158

Add a mutex to the region and take it in dev_store(), in the
block_bytes, start_block and blocks stores, and in dev_show(), since
they all look at hr_bdev_file. dev_store() sleeps with it held while it
waits for the heartbeat thread. That is fine because neither the thread
nor the teardown paths take it.

With the patch the same test ran for 10 minutes with lockdep on and
nothing was reported.

Fixes: a7f6a5fb4bde ("[PATCH] OCFS2: The Second Oracle Cluster Filesystem")
Reported-by: syzbot+5d4e96acd0febccb5e87@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=5d4e96acd0febccb5e87
Cc: stable@vger.kernel.org
Signed-off-by: Eklavya Singh <eklavyasingh2106@gmail.com>
---
 fs/ocfs2/cluster/heartbeat.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/ocfs2/cluster/heartbeat.c b/fs/ocfs2/cluster/heartbeat.c
index 1c3def99b..2ea870098 100644
--- a/fs/ocfs2/cluster/heartbeat.c
+++ b/fs/ocfs2/cluster/heartbeat.c
@@ -269,6 +269,8 @@ struct o2hb_region {
 	struct list_head	hr_handler_list;
 	/* Serializes timeout arming against failed-start and teardown. */
 	struct mutex		hr_arming_mutex;
+	/* Serializes the region's configfs attributes against dev_store(). */
+	struct mutex		hr_dev_mutex;
 	bool			hr_stopping;
 
 	/* last hb status, 0 for success, other value for error. */
@@ -1676,6 +1678,7 @@ static ssize_t o2hb_region_block_bytes_store(struct config_item *item,
 	unsigned long block_bytes;
 	unsigned int block_bits;
 
+	guard(mutex)(&reg->hr_dev_mutex);
 	if (reg->hr_bdev_file)
 		return -EINVAL;
 
@@ -1705,6 +1708,7 @@ static ssize_t o2hb_region_start_block_store(struct config_item *item,
 	char *p = (char *)page;
 	ssize_t ret;
 
+	guard(mutex)(&reg->hr_dev_mutex);
 	if (reg->hr_bdev_file)
 		return -EINVAL;
 
@@ -1731,6 +1735,7 @@ static ssize_t o2hb_region_blocks_store(struct config_item *item,
 	char *p = (char *)page;
 	int ret;
 
+	guard(mutex)(&reg->hr_dev_mutex);
 	if (reg->hr_bdev_file)
 		return -EINVAL;
 
@@ -1748,10 +1753,12 @@ static ssize_t o2hb_region_blocks_store(struct config_item *item,
 
 static ssize_t o2hb_region_dev_show(struct config_item *item, char *page)
 {
+	struct o2hb_region *reg = to_o2hb_region(item);
 	unsigned int ret = 0;
 
-	if (to_o2hb_region(item)->hr_bdev_file)
-		ret = sprintf(page, "%pg\n", reg_bdev(to_o2hb_region(item)));
+	guard(mutex)(&reg->hr_dev_mutex);
+	if (reg->hr_bdev_file)
+		ret = sprintf(page, "%pg\n", reg_bdev(reg));
 
 	return ret;
 }
@@ -1879,6 +1886,7 @@ static ssize_t o2hb_region_dev_store(struct config_item *item,
 	ssize_t ret = -EINVAL;
 	int live_threshold;
 
+	guard(mutex)(&reg->hr_dev_mutex);
 	if (reg->hr_bdev_file)
 		return -EINVAL;
 
@@ -2156,6 +2164,7 @@ static struct config_item *o2hb_heartbeat_group_make_item(struct config_group *g
 	reg->hr_key = crc32_le(reg->hr_region_num + O2NM_MAX_REGIONS,
 		name, strlen(name));
 	mutex_init(&reg->hr_arming_mutex);
+	mutex_init(&reg->hr_dev_mutex);
 	reg->hr_stopping = true;
 	INIT_DELAYED_WORK(&reg->hr_write_timeout_work, o2hb_write_timeout);
 	INIT_DELAYED_WORK(&reg->hr_nego_timeout_work, o2hb_nego_timeout);
-- 
2.55.0