FKIE_CVE-2026-45895

Vulnerability from fkie_nvd - Published: 2026-05-27 14:17 - Updated: 2026-05-27 14:48
Severity
Summary
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.
References
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/02bb1500f1479750e6557c8044f6a2d7e9d30c12
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/37ccd48cf35f3c8b9f2ea961a7b486b91eb71a82
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/414259caf81a397563fc9baca9c0ef856c4a97cf
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/53b2314b26b6640a3657cc924de63a1a8f26ac4d
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/77449e453dfc006ad738dec55374c4cbc056fd39
Impacted products
Vendor Product Version
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{
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nquota: fix livelock between quotactl and freeze_super\n\nWhen a filesystem is frozen, quotactl_block() enters a retry loop\nwaiting for the filesystem to thaw. It acquires s_umount, checks the\nfreeze state, drops s_umount and uses sb_start_write() - sb_end_write()\npair to wait for the unfreeze.\n\nHowever, this retry loop can trigger a livelock issue, specifically on\nkernels with preemption disabled.\n\nThe mechanism is as follows:\n1. freeze_super() sets SB_FREEZE_WRITE and calls sb_wait_write().\n2. sb_wait_write() calls percpu_down_write(), which initiates\n   synchronize_rcu().\n3. Simultaneously, quotactl_block() spins in its retry loop, immediately\n   executing the sb_start_write() - sb_end_write() pair.\n4. Because the kernel is non-preemptible and the loop contains no\n   scheduling points, quotactl_block() never yields the CPU. This\n   prevents that CPU from reaching an RCU quiescent state.\n5. synchronize_rcu() in the freezer thread waits indefinitely for the\n   quotactl_block() CPU to report a quiescent state.\n6. quotactl_block() spins indefinitely waiting for the freezer to\n   advance, which it cannot do as it is blocked on the RCU sync.\n\nThis results in a hang of the freezer process and 100% CPU usage by the\nquota process.\n\nWhile this can occur intermittently on multi-core systems, it is\nreliably reproducing on a node with the following script, running both\nthe freezer and the quota toggle on the same CPU:\n\n  # mkfs.ext4 -O quota /dev/sda 2g \u0026\u0026 mkdir a_mount\n  # mount /dev/sda -o quota,usrquota,grpquota a_mount\n  # taskset -c 3 bash -c \"while true; do xfs_freeze -f a_mount; \\\n    xfs_freeze -u a_mount; done\" \u0026\n  # taskset -c 3 bash -c \"while true; do quotaon a_mount; \\\n    quotaoff a_mount; done\" \u0026\n\nAdding cond_resched() to the retry loop fixes the issue. It acts as an\nRCU quiescent state, allowing synchronize_rcu() in percpu_down_write()\nto complete."
    }
  ],
  "id": "CVE-2026-45895",
  "lastModified": "2026-05-27T14:48:31.480",
  "metrics": {},
  "published": "2026-05-27T14:17:03.733",
  "references": [
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/02bb1500f1479750e6557c8044f6a2d7e9d30c12"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/37ccd48cf35f3c8b9f2ea961a7b486b91eb71a82"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/414259caf81a397563fc9baca9c0ef856c4a97cf"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/53b2314b26b6640a3657cc924de63a1a8f26ac4d"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/77449e453dfc006ad738dec55374c4cbc056fd39"
    }
  ],
  "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
  "vulnStatus": "Awaiting Analysis"
}


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