CVE-2026-89655 (GCVE-0-2026-89655)
Vulnerability from cvelistv5
Published
2026-09-11 19:45
Modified
2026-10-19 08:50
Severity ?
9.8 (Critical) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Summary
In the Linux kernel, the following vulnerability has been resolved: ceph: fix UAF in __kick_flushing_caps() on cf entry freed during unlock list_for_each_entry() iterates ci->i_cap_flush_list but drops i_ceph_lock to send cap messages. During the unlock window, handle_cap_flush_ack() can acquire i_ceph_lock, detach cf entries with tid <= flush_tid from the list, release i_ceph_lock, and free them via ceph_free_cap_flush() outside any lock. When the original thread reacquires i_ceph_lock and the for-loop macro advances via cf = list_next_entry(cf, i_list), it dereferences cf->i_list.next on freed memory. The race timeline: __kick_flushing_caps() handle_cap_flush_ack() ----------------------- ----------------------- holds i_ceph_lock <--- iterates to cf (tid=10) prepares FLUSH message drops i_ceph_lock <--- __send_cap() ── FLUSH(tid=10) MDS sends FLUSH_ACK(tid=10) ---> acquires i_ceph_lock cf->tid(10) <= flush_tid(10), detaches cf from i_cap_flush_list drops i_ceph_lock ceph_free_cap_flush(cf) <- frees it! acquires i_ceph_lock <--- for-loop advances: cf = list_next_entry(cf, i_list) -- UAF on freed cf->i_list.next The cf was just sent by __kick_flushing_caps itself via __send_cap(). The MDS may respond with FLUSH_ACK quickly enough that handle_cap_flush_ack() frees cf before __kick_flushing_caps can finish the iteration. Fix by converting to a manual while loop: save the next pointer under i_ceph_lock before dropping it, then use the saved pointer after reacquiring, so the potentially-freed cf is never accessed again.
References
Impacted products
Vendor Product Version
Linux Linux Version: e4500b5e35c213e0f97be7cb69328c0877203a79
Version: e4500b5e35c213e0f97be7cb69328c0877203a79
Version: e4500b5e35c213e0f97be7cb69328c0877203a79
Version: e4500b5e35c213e0f97be7cb69328c0877203a79
Version: e4500b5e35c213e0f97be7cb69328c0877203a79
Version: e4500b5e35c213e0f97be7cb69328c0877203a79
Version: e4500b5e35c213e0f97be7cb69328c0877203a79
Version: e4500b5e35c213e0f97be7cb69328c0877203a79
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          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nceph: fix UAF in __kick_flushing_caps() on cf entry freed during unlock\n\nlist_for_each_entry() iterates ci-\u003ei_cap_flush_list but drops\ni_ceph_lock to send cap messages.  During the unlock window,\nhandle_cap_flush_ack() can acquire i_ceph_lock, detach cf entries\nwith tid \u003c= flush_tid from the list, release i_ceph_lock, and free\nthem via ceph_free_cap_flush() outside any lock.  When the original\nthread reacquires i_ceph_lock and the for-loop macro advances via\ncf = list_next_entry(cf, i_list), it dereferences cf-\u003ei_list.next\non freed memory.\n\nThe race timeline:\n\n  __kick_flushing_caps()              handle_cap_flush_ack()\n  -----------------------             -----------------------\n  holds i_ceph_lock        \u003c---\n  iterates to cf (tid=10)\n  prepares FLUSH message\n  drops i_ceph_lock        \u003c---\n  __send_cap() \u2500\u2500 FLUSH(tid=10)\n\t                              MDS sends FLUSH_ACK(tid=10)\n                           ---\u003e       acquires i_ceph_lock\n                                      cf-\u003etid(10) \u003c= flush_tid(10),\n                                      detaches cf from i_cap_flush_list\n                                      drops i_ceph_lock\n                                      ceph_free_cap_flush(cf) \u003c- frees it!\n  acquires i_ceph_lock     \u003c---\n  for-loop advances:\n    cf = list_next_entry(cf, i_list)\n      -- UAF on freed cf-\u003ei_list.next\n\nThe cf was just sent by __kick_flushing_caps itself via __send_cap().\nThe MDS may respond with FLUSH_ACK quickly enough that\nhandle_cap_flush_ack() frees cf before __kick_flushing_caps can\nfinish the iteration.\n\nFix by converting to a manual while loop: save the next pointer\nunder i_ceph_lock before dropping it, then use the saved pointer\nafter reacquiring, so the potentially-freed cf is never accessed again."
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              "value": "AV:N - handle_cap_flush_ack() runs in the ceph-msgr kworker on CEPH_MSG_CLIENT_CAPS FLUSH_ACK from the MDS over TCP; a malicious or compromised MDS frees the ceph_cap_flush while __kick_flushing_caps() still holds that pointer after dropping i_ceph_lock to send FLUSH.\nAC:L - The MDS controls both sides of the race: it can induce kicking without s_mutex (reconnect sets CEPH_I_KICK_FLUSH, then delayed ceph_check_caps or revoke-queued writeback) and immediately send FLUSH_ACK for the known flush tid in the unlock window around __send_cap().\nPR:N - The attacker is the remote MDS peer on an already-connected kernel CephFS client and needs no Linux UID, capability, or init-namespace root on the victim, matching other MDS-driven client CVEs such as CVE-2026-89651.\nUI:N - Once CephFS is mounted (standard in Rook/Kubernetes/OpenStack), cap-flush kicking runs from mdsc delayed work and writeback, and FLUSH_ACK is processed automatically in kworker context with no further victim mount or interactive step at exploit time.\nS:U - The use-after-free is of a ceph_cap_flush object in the host kernel heap and stays inside the same kernel security authority; it does not cross a VM, IOMMU, or sandbox boundary.\nC:H - After ceph_free_cap_flush(), __kick_flushing_caps() does list_next_entry() on the freed cf; reclaiming the dedicated ceph_cap_flush slab with attacker-controlled objects yields a fake next pointer and arbitrary kernel read, scored High per UAF guidance.\nI:H - The iterator then treats the sprayed object as a live ceph_cap_flush (tid, caps, is_capsnap, i_list) and continues sending cap state from it, giving a heap-reuse write and control-flow hijack primitive scored High for use-after-free.\nA:H - Dereferencing freed cf-\u003ei_list.next oopses or panics the kicker thread even without full exploitation, and a malicious MDS can retrigger the race on every cap flush."
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    "cveId": "CVE-2026-89655",
    "datePublished": "2026-09-11T19:45:44.841Z",
    "dateReserved": "2026-09-11T19:38:34.743Z",
    "dateUpdated": "2026-10-19T08:50:26.879Z",
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it does not cross a VM, IOMMU, or sandbox boundary.\\nC:H - After ceph_free_cap_flush(), __kick_flushing_caps() does list_next_entry() on the freed cf; reclaiming the dedicated ceph_cap_flush slab with attacker-controlled objects yields a fake next pointer and arbitrary kernel read, scored High per UAF guidance.\\nI:H - The iterator then treats the sprayed object as a live ceph_cap_flush (tid, caps, is_capsnap, i_list) and continues sending cap state from it, giving a heap-reuse write and control-flow hijack primitive scored High for use-after-free.\\nA:H - Dereferencing freed cf-\u003ei_list.next oopses or panics the kicker thread even without full exploitation, and a malicious MDS can retrigger the race on every cap flush.\"}]}], \"affected\": [{\"repo\": \"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\", \"vendor\": \"Linux\", \"product\": \"Linux\", \"versions\": [{\"status\": \"affected\", \"version\": \"e4500b5e35c213e0f97be7cb69328c0877203a79\", \"lessThan\": \"091137821e1fc88f37e15201abf055c9494ddc61\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"e4500b5e35c213e0f97be7cb69328c0877203a79\", \"lessThan\": \"01542430081014d80fc9e70e92d6647432ddb7fc\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"e4500b5e35c213e0f97be7cb69328c0877203a79\", \"lessThan\": \"23eb34a53a53cb1a6dab1eeee830633207ac158d\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"e4500b5e35c213e0f97be7cb69328c0877203a79\", \"lessThan\": \"19f16f04c2b014a7dd214dc1e42557d8530b16f3\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"e4500b5e35c213e0f97be7cb69328c0877203a79\", \"lessThan\": \"2701431aa3cc8b23efe6890182e7b04f5e76fab5\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"e4500b5e35c213e0f97be7cb69328c0877203a79\", \"lessThan\": \"fe46746087b5b9c5bb2d022df6c7819218494ced\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"e4500b5e35c213e0f97be7cb69328c0877203a79\", \"lessThan\": \"2dba24dcd5050be4b7b119e6f0b01f62203b5d26\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"e4500b5e35c213e0f97be7cb69328c0877203a79\", \"lessThan\": \"7af4c4f01305b0935adf6d4301b1ec407025485d\", \"versionType\": \"git\"}], \"programFiles\": [\"fs/ceph/caps.c\"], \"defaultStatus\": \"unaffected\"}, {\"repo\": \"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\", \"vendor\": \"Linux\", \"product\": \"Linux\", \"versions\": [{\"status\": \"affected\", \"version\": \"4.8\"}, {\"status\": \"unaffected\", \"version\": \"0\", \"lessThan\": \"4.8\", \"versionType\": \"semver\"}, {\"status\": \"unaffected\", \"version\": \"5.10.270\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"5.10.*\"}, {\"status\": \"unaffected\", \"version\": \"5.15.221\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"5.15.*\"}, {\"status\": \"unaffected\", \"version\": \"6.1.188\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"6.1.*\"}, {\"status\": \"unaffected\", \"version\": \"6.6.157\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"6.6.*\"}, {\"status\": \"unaffected\", \"version\": \"6.12.109\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"6.12.*\"}, {\"status\": \"unaffected\", \"version\": \"6.18.50\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"6.18.*\"}, {\"status\": \"unaffected\", \"version\": \"7.2.4\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"7.2.*\"}, {\"status\": \"unaffected\", \"version\": \"7.3\", \"versionType\": \"original_commit_for_fix\", \"lessThanOrEqual\": \"*\"}], \"programFiles\": [\"fs/ceph/caps.c\"], \"defaultStatus\": \"affected\"}], \"references\": [{\"url\": \"https://git.kernel.org/stable/c/091137821e1fc88f37e15201abf055c9494ddc61\"}, {\"url\": \"https://git.kernel.org/stable/c/01542430081014d80fc9e70e92d6647432ddb7fc\"}, {\"url\": \"https://git.kernel.org/stable/c/23eb34a53a53cb1a6dab1eeee830633207ac158d\"}, {\"url\": \"https://git.kernel.org/stable/c/19f16f04c2b014a7dd214dc1e42557d8530b16f3\"}, {\"url\": \"https://git.kernel.org/stable/c/2701431aa3cc8b23efe6890182e7b04f5e76fab5\"}, {\"url\": \"https://git.kernel.org/stable/c/fe46746087b5b9c5bb2d022df6c7819218494ced\"}, {\"url\": \"https://git.kernel.org/stable/c/2dba24dcd5050be4b7b119e6f0b01f62203b5d26\"}, {\"url\": \"https://git.kernel.org/stable/c/7af4c4f01305b0935adf6d4301b1ec407025485d\"}], \"x_generator\": {\"engine\": \"bippy-1.2.0\"}, \"descriptions\": [{\"lang\": \"en\", \"value\": \"In the Linux kernel, the following vulnerability has been resolved:\\n\\nceph: fix UAF in __kick_flushing_caps() on cf entry freed during unlock\\n\\nlist_for_each_entry() iterates ci-\u003ei_cap_flush_list but drops\\ni_ceph_lock to send cap messages.  During the unlock window,\\nhandle_cap_flush_ack() can acquire i_ceph_lock, detach cf entries\\nwith tid \u003c= flush_tid from the list, release i_ceph_lock, and free\\nthem via ceph_free_cap_flush() outside any lock.  When the original\\nthread reacquires i_ceph_lock and the for-loop macro advances via\\ncf = list_next_entry(cf, i_list), it dereferences cf-\u003ei_list.next\\non freed memory.\\n\\nThe race timeline:\\n\\n  __kick_flushing_caps()              handle_cap_flush_ack()\\n  -----------------------             -----------------------\\n  holds i_ceph_lock        \u003c---\\n  iterates to cf (tid=10)\\n  prepares FLUSH message\\n  drops i_ceph_lock        \u003c---\\n  __send_cap() \\u2500\\u2500 FLUSH(tid=10)\\n\\t                              MDS sends FLUSH_ACK(tid=10)\\n                           ---\u003e       acquires i_ceph_lock\\n                                      cf-\u003etid(10) \u003c= flush_tid(10),\\n                                      detaches cf from i_cap_flush_list\\n                                      drops i_ceph_lock\\n                                      ceph_free_cap_flush(cf) \u003c- frees it!\\n  acquires i_ceph_lock     \u003c---\\n  for-loop advances:\\n    cf = list_next_entry(cf, i_list)\\n      -- UAF on freed cf-\u003ei_list.next\\n\\nThe cf was just sent by __kick_flushing_caps itself via __send_cap().\\nThe MDS may respond with FLUSH_ACK quickly enough that\\nhandle_cap_flush_ack() frees cf before __kick_flushing_caps can\\nfinish the iteration.\\n\\nFix by converting to a manual while loop: save the next pointer\\nunder i_ceph_lock before dropping it, then use the saved pointer\\nafter reacquiring, so the potentially-freed cf is never accessed again.\"}], \"cpeApplicability\": [{\"nodes\": [{\"negate\": false, \"cpeMatch\": [{\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"5.10.270\", \"versionStartIncluding\": \"4.8\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"5.15.221\", \"versionStartIncluding\": \"4.8\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"6.1.188\", \"versionStartIncluding\": \"4.8\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"6.6.157\", \"versionStartIncluding\": \"4.8\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"6.12.109\", \"versionStartIncluding\": \"4.8\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"6.18.50\", \"versionStartIncluding\": \"4.8\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"7.2.4\", \"versionStartIncluding\": \"4.8\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"7.3\", \"versionStartIncluding\": \"4.8\"}], \"operator\": \"OR\"}]}], \"providerMetadata\": {\"orgId\": \"416baaa9-dc9f-4396-8d5f-8c081fb06d67\", \"shortName\": \"Linux\", \"dateUpdated\": \"2026-10-19T08:50:26.879Z\"}}}",
      "cveMetadata": "{\"cveId\": \"CVE-2026-89655\", \"state\": \"PUBLISHED\", \"dateUpdated\": \"2026-10-19T08:50:26.879Z\", \"dateReserved\": \"2026-09-11T19:38:34.743Z\", \"assignerOrgId\": \"416baaa9-dc9f-4396-8d5f-8c081fb06d67\", \"datePublished\": \"2026-09-11T19:45:44.841Z\", \"assignerShortName\": \"Linux\"}",
      "dataType": "CVE_RECORD",
      "dataVersion": "5.2"
    }
  }
}


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