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:
- dquot are added to releasing_dquots list during regular operations.
- FS Freeze starts, however, this does not flush the quota_release_work queue.
- Freeze completes.
- Kernel eventually tries to flush the workqueue while FS is frozen which
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.