CVE-2024-56780 — quota: flush quota_release_work upon quota writeback

Affected: Linux

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.

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CVE-2024-56780 — quota: flush quota_release_work upon quota writeback