GHSA-M843-78WR-M386

Vulnerability from github – Published: 2026-09-11 21:31 – Updated: 2026-10-30 15:33
VLAI
Details

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

ceph: do not repeat ceph_trim_dentries() if no progress possible

ceph_cap_reclaim_work() re-queues itself for as long as ceph_trim_dentries() returns -EAGAIN, which happens whenever a lease walk exhausts its nr_to_scan budget. This creates a busy loop that consumes CPU without making any progress when there is nothing to reclaim: with no cap pressure (count==0) and every scanned lease still valid, each pass runs the full scan budget down to zero and returns -EAGAIN, only to be queued again immediately.

The dir-lease walk made this worse. When expire_dir_lease is false (i.e. we have no intention of reclaiming dir leases), __dir_lease_check() returned TOUCH for every valid lease. TOUCH moves the dentry to the tail of the list and resets di->time via __dentry_dir_lease_touch(), so a walk over N valid leases pointlessly rewrote the list, refreshed the timestamps (preventing them from ever aging out) and always drained nr_to_scan, guaranteeing the -EAGAIN requeue.

Fix this in three steps:

  • Return KEEP instead of TOUCH when expire_dir_lease is false. If we are not going to reclaim the lease, leave it in place instead of churning the list and resetting its timestamp; the walk then terminates naturally (or via STOP at the first fresh lease).

  • Only return -EAGAIN from the first (dentry-lease) walk when something was actually freed. A full batch that frees nothing means retrying the same list immediately is futile; fall through to the dir-lease walk instead.

  • After both walks, bail out with success (0) when nothing was freed and there is no cap pressure (count==0). There is no reason to keep retrying when we are not over the cap limit and made no progress.

Under real cap pressure (count>0) the reclaim path is unchanged and still retries via -EAGAIN.

Without this patch, I saw 500 ceph_trim_dentries() calls per second on our web servers. This is very visible in /proc/lock_stat (5 minute capture):

          class name    con-bounces    contentions   waittime-min   waittime-max waittime-total   waittime-avg    acq-bounces   acquisitions   holdtime-min   holdtime-max holdtime-total   holdtime-avg

&mdsc->dentry_list_lock: 126180 128218 0.04 8063.44 15986965.20 124.69 1573354 5296812 0.04 8291.28 74164526.48 14.00

&mdsc->dentry_list_lock 111736 [<000000007b11e319>] __ceph_dentry_dir_lease_touch+0x7c/0xa8 &mdsc->dentry_list_lock 2631 [<0000000050597999>] __dentry_leases_walk+0x64/0x2c8 &mdsc->dentry_list_lock 3878 [<00000000c0022f62>] __ceph_dentry_lease_touch+0x5c/0xa8 &mdsc->dentry_list_lock 9973 [<000000002f27cb6f>] __dentry_lease_unlist+0x50/0xa0

&mdsc->dentry_list_lock 123621 [<0000000050597999>] __dentry_leases_walk+0x64/0x2c8 &mdsc->dentry_list_lock 1822 [<000000007b11e319>] __ceph_dentry_dir_lease_touch+0x7c/0xa8 &mdsc->dentry_list_lock 2720 [<000000002f27cb6f>] __dentry_lease_unlist+0x50/0xa0 &mdsc->dentry_list_lock 55 [<00000000c0022f62>] __ceph_dentry_lease_touch+0x5c/0xa8

With this patch:

          class name    con-bounces    contentions   waittime-min   waittime-max waittime-total   waittime-avg    acq-bounces   acquisitions   holdtime-min   holdtime-max holdtime-total   holdtime-avg

&mdsc->dentry_list_lock: 1203 1215 0.16 408.88 33082.88 27.23 4320501 7357389 0.04 500.64 1961578.00 0.27

&mdsc->dentry_list_lock 1029 [<000000003c9aea8a>] __ceph_dentry_dir_lease_touch+0x7c/0xa8 &mdsc->dentry_list_lock 1 ---truncated---

Severity
7.5 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Show details on source website
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{
  "affected": [],
  "aliases": [
    "CVE-2026-89647"
  ],
  "database_specific": {
    "cwe_ids": [],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2026-09-11T20:19:50Z",
    "severity": "HIGH"
  },
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nceph: do not repeat ceph_trim_dentries() if no progress possible\n\nceph_cap_reclaim_work() re-queues itself for as long as\nceph_trim_dentries() returns -EAGAIN, which happens whenever a lease\nwalk exhausts its `nr_to_scan` budget.  This creates a busy loop that\nconsumes CPU without making any progress when there is nothing to\nreclaim: with no cap pressure (`count==0`) and every scanned lease\nstill valid, each pass runs the full scan budget down to zero and\nreturns `-EAGAIN`, only to be queued again immediately.\n\nThe dir-lease walk made this worse.  When `expire_dir_lease` is\n`false` (i.e. we have no intention of reclaiming dir leases),\n__dir_lease_check() returned `TOUCH` for every valid lease.  `TOUCH`\nmoves the dentry to the tail of the list and resets `di-\u003etime` via\n__dentry_dir_lease_touch(), so a walk over N valid leases pointlessly\nrewrote the list, refreshed the timestamps (preventing them from ever\naging out) and always drained `nr_to_scan`, guaranteeing the `-EAGAIN`\nrequeue.\n\nFix this in three steps:\n\n - Return `KEEP` instead of `TOUCH` when `expire_dir_lease` is\n   `false`.  If we are not going to reclaim the lease, leave it in\n   place instead of churning the list and resetting its timestamp; the\n   walk then terminates naturally (or via `STOP` at the first fresh\n   lease).\n\n - Only return `-EAGAIN` from the first (dentry-lease) walk when something\n   was actually freed.  A full batch that frees nothing means retrying\n   the same list immediately is futile; fall through to the dir-lease\n   walk instead.\n\n - After both walks, bail out with success (0) when nothing was freed\n   and there is no cap pressure (`count==0`).  There is no reason to\n   keep retrying when we are not over the cap limit and made no\n   progress.\n\nUnder real cap pressure (`count\u003e0`) the reclaim path is unchanged and\nstill retries via `-EAGAIN`.\n\nWithout this patch, I saw 500 ceph_trim_dentries() calls per second on\nour web servers.  This is very visible in `/proc/lock_stat` (5 minute\ncapture):\n\n              class name    con-bounces    contentions   waittime-min   waittime-max waittime-total   waittime-avg    acq-bounces   acquisitions   holdtime-min   holdtime-max holdtime-total   holdtime-avg\n\n \u0026mdsc-\u003edentry_list_lock:        126180         128218           0.04        8063.44    15986965.20         124.69        1573354        5296812           0.04        8291.28    74164526.48          14.00\n -----------------------\n \u0026mdsc-\u003edentry_list_lock         111736          [\u003c000000007b11e319\u003e] __ceph_dentry_dir_lease_touch+0x7c/0xa8\n \u0026mdsc-\u003edentry_list_lock           2631          [\u003c0000000050597999\u003e] __dentry_leases_walk+0x64/0x2c8\n \u0026mdsc-\u003edentry_list_lock           3878          [\u003c00000000c0022f62\u003e] __ceph_dentry_lease_touch+0x5c/0xa8\n \u0026mdsc-\u003edentry_list_lock           9973          [\u003c000000002f27cb6f\u003e] __dentry_lease_unlist+0x50/0xa0\n -----------------------\n \u0026mdsc-\u003edentry_list_lock         123621          [\u003c0000000050597999\u003e] __dentry_leases_walk+0x64/0x2c8\n \u0026mdsc-\u003edentry_list_lock           1822          [\u003c000000007b11e319\u003e] __ceph_dentry_dir_lease_touch+0x7c/0xa8\n \u0026mdsc-\u003edentry_list_lock           2720          [\u003c000000002f27cb6f\u003e] __dentry_lease_unlist+0x50/0xa0\n \u0026mdsc-\u003edentry_list_lock             55          [\u003c00000000c0022f62\u003e] __ceph_dentry_lease_touch+0x5c/0xa8\n\nWith this patch:\n\n              class name    con-bounces    contentions   waittime-min   waittime-max waittime-total   waittime-avg    acq-bounces   acquisitions   holdtime-min   holdtime-max holdtime-total   holdtime-avg\n\n \u0026mdsc-\u003edentry_list_lock:          1203           1215           0.16         408.88       33082.88          27.23        4320501        7357389           0.04         500.64     1961578.00           0.27\n -----------------------\n \u0026mdsc-\u003edentry_list_lock           1029          [\u003c000000003c9aea8a\u003e] __ceph_dentry_dir_lease_touch+0x7c/0xa8\n \u0026mdsc-\u003edentry_list_lock            1\n---truncated---",
  "id": "GHSA-m843-78wr-m386",
  "modified": "2026-10-30T15:33:23Z",
  "published": "2026-09-11T21:31:35Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-89647"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/37d6edb2f03b29399a3a337fae78674de51e1695"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/3d122b2feb1dd76bb5041bdea5e1e1b007d8d415"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/5a541eb401acb89d791da41189d7a79220164b93"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/dd2e9824e5e7dba406005ff779a107f773175a76"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/e7d7aa7b730178278109c41fa1b17b06873065d5"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/f2f6daf8e99e15d636b1706bb4efd9a856195324"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ]
}



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