CVE-2026-72034 (GCVE-0-2026-72034)

Vulnerability from cvelistv5 – Published: 2026-08-15 05:51 – Updated: 2026-08-17 05:39
VLAI
Title
fhandle: reject detached mounts in capable_wrt_mount()
Summary
In the Linux kernel, the following vulnerability has been resolved: fhandle: reject detached mounts in capable_wrt_mount() The recent fhandle RCU fix moved the mount namespace capability check into capable_wrt_mount(), so a non-NULL mnt_namespace survives the ns_capable() dereference. The helper still assumes the later READ_ONCE(mount->mnt_ns) must be non-NULL because may_decode_fh() checked is_mounted() first. That assumption is not stable. A detached mount from open_tree(..., OPEN_TREE_CLONE) can be dissolved on fput while open_by_handle_at() is between those checks, and umount_tree() can clear mount->mnt_ns. If the helper observes NULL, it dereferences mnt_ns->user_ns and panics. Return false when the RCU read observes a detached mount. This keeps the relaxed permission path conservative: a mount no longer attached to a namespace cannot authorize open_by_handle_at() access.
Severity
7.8 (High)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Assigner
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: 620c266f394932e5decc4b34683a75dfc59dc2f4 , < 15104234c267ebe04b9f9a73e5c2179cc265ff60 (git)
Affected: 620c266f394932e5decc4b34683a75dfc59dc2f4 , < 6c52226072a3c61337b1eec1799bb987748884fa (git)
Affected: 620c266f394932e5decc4b34683a75dfc59dc2f4 , < 6ee183d89261bf1d1cf9f06d80a40dab8f36ee55 (git)
Affected: 620c266f394932e5decc4b34683a75dfc59dc2f4 , < 6c732471740bc2ac9b0946134f9f551dc75f4369 (git)
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Linux Linux Affected: 6.11
Unaffected: 0 , < 6.11 (semver)
Unaffected: 6.12.97 , ≤ 6.12.* (semver)
Unaffected: 6.18.40 , ≤ 6.18.* (semver)
Unaffected: 7.1.5 , ≤ 7.1.* (semver)
Unaffected: 7.2 , ≤ * (original_commit_for_fix)
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          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nfhandle: reject detached mounts in capable_wrt_mount()\n\nThe recent fhandle RCU fix moved the mount namespace capability check\ninto capable_wrt_mount(), so a non-NULL mnt_namespace survives the\nns_capable() dereference. The helper still assumes the later\nREAD_ONCE(mount-\u003emnt_ns) must be non-NULL because may_decode_fh()\nchecked is_mounted() first.\n\nThat assumption is not stable. A detached mount from\nopen_tree(..., OPEN_TREE_CLONE) can be dissolved on fput while\nopen_by_handle_at() is between those checks, and umount_tree() can\nclear mount-\u003emnt_ns. If the helper observes NULL, it dereferences\nmnt_ns-\u003euser_ns and panics.\n\nReturn false when the RCU read observes a detached mount. This keeps\nthe relaxed permission path conservative: a mount no longer attached\nto a namespace cannot authorize open_by_handle_at() access."
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            "baseSeverity": "HIGH",
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              "value": "AV:L - Exploitation requires local syscalls only: open_tree(OPEN_TREE_CLONE) to obtain a detached mount fd, open_by_handle_at() to enter may_decode_fh()/capable_wrt_mount(), and close()/fput to dissolve the mount; no network, adjacent, or physical interface reaches this VFS fhandle path.\nAC:L - The attacker controls both sides of the TOCTOU race by concurrently calling open_by_handle_at() on a detached mount fd while another thread closes that fd (or a dup) to trigger dissolve_on_fput()/umount_tree() clearing mount-\u003emnt_ns, making success depend on attacker timing rather than external state.\nPR:L - The relaxed permission path requires CAP_SYS_ADMIN in the current mount namespace (may_mount/open_tree) and CAP_DAC_READ_SEARCH in the current user namespace while lacking init_user_ns CAP_DAC_READ_SEARCH; these are obtainable by an unprivileged local user via user/mount namespaces (unshare -Urn), not real init-namespace root.\nUI:N - No victim action is required; the attacker creates the detached mount, supplies the file handle, and drives the concurrent open_by_handle_at()/close race entirely through their own syscalls and threads.\nS:U - Impact is confined to kernel memory/availability within the same security authority (local/container-triggered kernel panic or namespace lifetime corruption); it does not cross a VM escape, sandbox, or IOMMU boundary per kernel CVSS scope guidance.\nC:H - The affected window (since 620c266f3949) includes unsynchronized mount::mnt_ns access that can UAF freed mnt_namespace memory before the RCU fix, and later NULL/stale namespace dereferences; per kernel guidance UAF and mount-namespace lifetime races enable arbitrary kernel memory disclosure.\nI:H - Concurrent umount_tree() clearing mount-\u003emnt_ns while capable_wrt_mount() dereferences mnt_ns-\u003euser_ns is a mount-namespace lifetime corruption bug; UAF in the same code path can be leveraged for heap corruption and arbitrary kernel writes/code execution, not merely a benign error return.\nA:H - Observing NULL or freed mnt_ns during ns_capable(mnt_ns-\u003euser_ns, CAP_SYS_ADMIN) causes a kernel oops/panic; the race is repeatable by reopening detached mounts, enabling reliable host DoS from containerized tenants on multi-tenant cloud nodes."
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        "shortName": "Linux"
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    "datePublished": "2026-08-15T05:51:55.739Z",
    "dateReserved": "2026-08-09T03:40:39.901Z",
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      "cve": "CVE-2026-72034",
      "id": "CVE-2026-72034",
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      "product_status:known_affected": "184",
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      "source": "Red Hat CSAF VEX",
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      "title": "kernel: fhandle: reject detached mounts in capable_wrt_mount()",
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      "id": "CVE-2026-72034",
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      "product_status:known_affected": "29",
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      "source": "SUSE CSAF VEX",
      "status": "interim",
      "title": "SUSE CVE CVE-2026-72034",
      "url": "https://ftp.suse.com/pub/projects/security/csaf-vex/cve-2026-72034.json",
      "version": "4"
    }
  }
}



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