Re: [PATCH] fs: Fix freeze_bdev()/thaw_bdev() accounting of bd_fsfreeze_sb

Bob Peterson
2021-01-07T16:29:10Z

----- Original Message -----
> Can someone pick this up? Maybe through Jens' block tree as that is
> where my commit this is fixing up came from.
Christoph and Al,

Here is my version:

Bob Peterson

fs: fix freeze count problem in freeze_bdev

Before this patch, if you tried to freeze a device (function freeze_bdev)
while it was being unmounted, it would get NULL back from get_active_super
and correctly bypass the freeze calls. Unfortunately, it forgot to decrement
its bd_fsfreeze_count. Subsequent calls to device thaw (thaw_bdev) would
see the non-zero bd_fsfreeze_count and assume the bd_fsfreeze_sb value was
still valid. That's not a safe assumption and resulted in use-after-free,
which often caused fatal kernel errors like: "unable to handle page fault
for address."

This patch adds the necessary decrement of bd_fsfreeze_count for that
error path. It also adds code to set the bd_fsfreeze_sb to NULL when the
last reference is reached in thaw_bdev.

Reviewed-by: Bob Peterson <rpeterso@redhat.com>
---
fs/block_dev.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 9e56ee1f2652..c6daf7d12546 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -555,8 +555,10 @@ int freeze_bdev(struct block_device *bdev)
goto done;

sb = get_active_super(bdev);
- if (!sb)
+ if (!sb) {
+ bdev->bd_fsfreeze_count--;
goto sync;
+ }
if (sb->s_op->freeze_super)
error = sb->s_op->freeze_super(sb);
else
@@ -600,6 +602,7 @@ int thaw_bdev(struct block_device *bdev)
if (!sb)
goto out;

+ bdev->bd_fsfreeze_sb = NULL;
if (sb->s_op->thaw_super)
error = sb->s_op->thaw_super(sb);
else