PT-2026-43762 · Linux · Linux

Published

2026-05-27

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Updated

2026-05-27

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CVE-2026-45895

None

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In the Linux kernel, the following vulnerability has been resolved:
quota: fix livelock between quotactl and freeze super
When a filesystem is frozen, quotactl block() enters a retry loop waiting for the filesystem to thaw. It acquires s umount, checks the freeze state, drops s umount and uses sb start write() - sb end write() pair to wait for the unfreeze.
However, this retry loop can trigger a livelock issue, specifically on kernels with preemption disabled.
The mechanism is as follows:
  1. freeze super() sets SB FREEZE WRITE and calls sb wait write().
  2. sb wait write() calls percpu down write(), which initiates synchronize rcu().
  3. Simultaneously, quotactl block() spins in its retry loop, immediately executing the sb start write() - sb end write() pair.
  4. Because the kernel is non-preemptible and the loop contains no scheduling points, quotactl block() never yields the CPU. This prevents that CPU from reaching an RCU quiescent state.
  5. synchronize rcu() in the freezer thread waits indefinitely for the quotactl block() CPU to report a quiescent state.
  6. quotactl block() spins indefinitely waiting for the freezer to advance, which it cannot do as it is blocked on the RCU sync.
This results in a hang of the freezer process and 100% CPU usage by the quota process.
While this can occur intermittently on multi-core systems, it is reliably reproducing on a node with the following script, running both the freezer and the quota toggle on the same CPU:

mkfs.ext4 -O quota /dev/sda 2g && mkdir a mount

mount /dev/sda -o quota,usrquota,grpquota a mount

taskset -c 3 bash -c "while true; do xfs freeze -f a mount;

xfs freeze -u a mount; done" &

taskset -c 3 bash -c "while true; do quotaon a mount;

quotaoff a mount; done" &
Adding cond resched() to the retry loop fixes the issue. It acts as an RCU quiescent state, allowing synchronize rcu() in percpu down write() to complete.

Related Identifiers

CVE-2026-45895

Affected Products

Linux