5.5 - CVE-2024-56780

In the Linux kernel, the following vulnerability has been resolved:

quota: flush quota_release_work upon quota writeback

One of the paths quota writeback is called from is:

freeze_super()
sync_filesystem()
ext4_sync_fs()
dquot_writeback_dquots()

Since we currently don't always flush the quota_release_work queue in this path, we can end up with the following race:

1. dquot are added to releasing_dquots list during regular operations.
2. FS Freeze starts, however, this does not flush the quota_release_work queue.
3. Freeze completes.
4. Kernel eventually tries to flush the workqueue while FS is frozen which
hits a WARN_ON since transaction gets started during frozen state:

ext4_journal_check_start+0x28/0x110 [ext4] (unreliable)
__ext4_journal_start_sb+0x64/0x1c0 [ext4]
ext4_release_dquot+0x90/0x1d0 [ext4]
quota_release_workfn+0x43c/0x4d0

Which is the following line:

WARN_ON(sb->s_writers.frozen == SB_FREEZE_COMPLETE);

Which ultimately results in generic/390 failing due to dmesg noise. This was detected on powerpc machine 15 cores.

To avoid this, make sure to flush the workqueue during dquot_writeback_dquots() so we dont have any pending workitems after freeze.