256 lines
17 KiB
JSON
256 lines
17 KiB
JSON
{
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"dataType": "CVE_RECORD",
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"dataVersion": "5.2",
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"cveMetadata": {
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"cveId": "CVE-2026-89647",
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"assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
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"state": "PUBLISHED",
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"assignerShortName": "Linux",
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"dateReserved": "2026-09-11T19:38:34.741Z",
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"datePublished": "2026-09-11T19:45:38.593Z",
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"dateUpdated": "2026-10-30T12:32:29.127Z"
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},
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"cna": {
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"orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
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"shortName": "Linux",
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"dateUpdated": "2026-10-30T12:32:29.127Z"
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},
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"descriptions": [
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{
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"lang": "en",
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"value": "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->time` 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>0`) 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 &mdsc->dentry_list_lock: 126180 128218 0.04 8063.44 15986965.20 124.69 1573354 5296812 0.04 8291.28 74164526.48 14.00\n -----------------------\n &mdsc->dentry_list_lock 111736 [<000000007b11e319>] __ceph_dentry_dir_lease_touch+0x7c/0xa8\n &mdsc->dentry_list_lock 2631 [<0000000050597999>] __dentry_leases_walk+0x64/0x2c8\n &mdsc->dentry_list_lock 3878 [<00000000c0022f62>] __ceph_dentry_lease_touch+0x5c/0xa8\n &mdsc->dentry_list_lock 9973 [<000000002f27cb6f>] __dentry_lease_unlist+0x50/0xa0\n -----------------------\n &mdsc->dentry_list_lock 123621 [<0000000050597999>] __dentry_leases_walk+0x64/0x2c8\n &mdsc->dentry_list_lock 1822 [<000000007b11e319>] __ceph_dentry_dir_lease_touch+0x7c/0xa8\n &mdsc->dentry_list_lock 2720 [<000000002f27cb6f>] __dentry_lease_unlist+0x50/0xa0\n &mdsc->dentry_list_lock 55 [<00000000c0022f62>] __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 &mdsc->dentry_list_lock: 1203 1215 0.16 408.88 33082.88 27.23 4320501 7357389 0.04 500.64 1961578.00 0.27\n -----------------------\n &mdsc->dentry_list_lock 1029 [<000000003c9aea8a>] __ceph_dentry_dir_lease_touch+0x7c/0xa8\n &mdsc->dentry_list_lock 1\n---truncated---"
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}
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],
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"metrics": [
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{
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"cvssV3_1": {
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"version": "3.1",
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"vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
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"baseScore": 7.5,
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"baseSeverity": "HIGH"
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},
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"scenarios": [
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{
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"lang": "en",
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"value": "AV:N - fs/ceph registers ceph_export_ops, so remote nfsd/ksmbd readdir and lookup on an exported CephFS mount populate dentry and dir-lease lists and drive ceph_trim_dentries() via the cap-reclaim workqueue; a compromised MDS on the existing client TCP session can also keep those leases valid.\nAC:L - Default caps_max leaves count==0, so expire_dir_lease is false and valid dir leases are TOUCHed until nr_to_scan hits zero; ceph_cap_reclaim_work() then immediately requeues on -EAGAIN. An attacker who walks or creates many dentries, or a peer that grants many still-valid leases, triggers this deterministically with no race.\nPR:N - In the network-export scenario the attacker needs no local account or capability on the victim host; guest or anonymous NFS/SMB exports of CephFS, and a remote MDS peer on an already-connected kernel client, are reasonable high-severity deployments matching other Ceph client CVEs.\nUI:N - After CephFS is mounted and optionally exported, delayed_work periodically queues reclaim and the attacker’s own readdir/lookup or MDS lease traffic is enough; no separate victim mount, open, or confirmation is required at exploit time.\nS:U - The busy loop, CPU burn, and dentry_list_lock contention stay inside the host kernel CephFS client and do not cross a VM, IOMMU, or sandbox boundary to another security authority.\nC:N - The defect only rescans and TOUCHes still-valid dentry leases; it does not perform an out-of-bounds read, use-after-free, or any other kernel-memory disclosure.\nI:N - The walk only reshuffles lease-list order and refreshes dentry timestamps; there is no memory corruption, arbitrary write, or control-flow hijack primitive.\nA:H - ceph_cap_reclaim_work() immediately requeues itself on -EAGAIN, forming an unbounded busy loop on the max_active=1 ceph-cap workqueue (observed ~500 iterations/s) that starves cap-release work, burns CPU, and contends dentry_list_lock enough to stall Ceph directory operations."
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}
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]
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}
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],
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"product": "Linux",
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"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
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"fs/ceph/dir.c"
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"title": "ceph: do not repeat ceph_trim_dentries() if no progress possible",
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"id": "CVE-2026-89647",
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"role": "CISA Coordinator",
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} |