CVE-2026-89762 (GCVE-0-2026-89762)
Vulnerability from cvelistv5
Published
2026-09-11 19:47
Modified
2026-10-21 12:48
Severity ?
7.8 (High) - CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Summary
In the Linux kernel, the following vulnerability has been resolved: apparmor: fix cred UAF caused by begin_current_label_crit_section() AppArmor's begin_current_label_crit_section() is a scary function called from lots of LSM hooks (in particular VFS/socket-related ones) that checks if the label referenced by the current creds is marked FLAG_STALE, and if so, attempts to use aa_replace_current_label() to replace the creds with an updated version that uses a new label. The first problem with this is that it would directly lead to UAF of `struct cred` if anything in the kernel takes a pointer to the current creds and accesses these past a security hook invocation that replaces creds, like so: ``` const struct cred *cred = current_cred(); alloc_file_pseudo(...); uid_t uid = cred->euid; ``` I don't know if anything in the kernel actually does this, but I think it is very surprising that this pattern could lead to UAF. The second problem is that things go wrong when aa_replace_current_label() runs with overridden credentials. aa_replace_current_label() bails out if `current_cred() != current_real_cred()` (mirroring the check in proc_pid_attr_write()), but this check can't actually reliably detect overridden credentials because the overridden creds can be the same as the objective creds. So in approximately the following scenario, things go wrong: 1. task begins with <creds A> (as both objective and subjective creds), with refcount=2 2. task grabs an extra reference on <creds A> for overriding 3. task calls override_creds(<creds A>), which returns a pointer to the old subjective creds (<creds A>) 4. task enters AppArmor LSM hook 5. AppArmor checks that objective/subjective creds are equal 6. AppArmor replaces both cred pointers with <creds B> and drops 2 refs on <creds A> 7. task leaves AppArmor LSM hook 8. task calls revert_creds(<creds A>) 9. now task->cred is <creds A> while task->real_cred is <creds B>, but the task_struct logically holds two references to <creds B> 10. another task drops the extra reference on <creds A> that was used for overriding, refcount drops to 0 11. now task->real_cred points to freed creds At this point, any access to current_cred() will be UAF. I have a test case where I run aa-disable on a profile while a process using that profile is blocked on splice() from a FUSE passthrough file into a full pipe; after the profile update, the pipe becomes empty, splice() resumes, the credentials go out of sync, and a subsequent getuid() syscall results in a KASAN UAF splat. To fix this, instead of directly replacing creds, do it via task_work that will run at the end of the current syscall. (The point in time at which the cred replacement happens should have no correctness impact; it is just a performance optimization to avoid unnecessarily touching the refcount of the new label.) Note that AppArmor still performs direct cred replacements in the sb_pivotroot LSM hook after this change, and that direct cred replacements can still happen in VFS ->write() callbacks via proc_pid_attr_write(). There are two options for what to do with aa_dup_task_ctx(): Either explicitly reset new->label_replacement_pending after the entire aa_task_ctx has been copied, or switch to manually copying members over. I am switching to manually copying members over because that should make bugs more obvious.
References
Impacted products
Vendor Product Version
Linux Linux Version: c75afcd153f6147d3b094f45a1d87e5df7f4f053
Version: c75afcd153f6147d3b094f45a1d87e5df7f4f053
Version: c75afcd153f6147d3b094f45a1d87e5df7f4f053
Version: c75afcd153f6147d3b094f45a1d87e5df7f4f053
Version: c75afcd153f6147d3b094f45a1d87e5df7f4f053
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{
  "containers": {
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          "product": "Linux",
          "programFiles": [
            "security/apparmor/include/cred.h",
            "security/apparmor/include/task.h",
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            "security/apparmor/include/task.h",
            "security/apparmor/task.c"
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          "vendor": "Linux",
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              "version": "7.2.4",
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              "version": "7.3",
              "versionType": "original_commit_for_fix"
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      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\napparmor: fix cred UAF caused by begin_current_label_crit_section()\n\nAppArmor\u0027s begin_current_label_crit_section() is a scary function called\nfrom lots of LSM hooks (in particular VFS/socket-related ones) that checks\nif the label referenced by the current creds is marked FLAG_STALE, and if\nso, attempts to use aa_replace_current_label() to replace the creds with an\nupdated version that uses a new label.\n\nThe first problem with this is that it would directly lead to UAF of\n`struct cred` if anything in the kernel takes a pointer to the current\ncreds and accesses these past a security hook invocation that replaces\ncreds, like so:\n```\nconst struct cred *cred = current_cred();\nalloc_file_pseudo(...);\nuid_t uid = cred-\u003eeuid;\n```\nI don\u0027t know if anything in the kernel actually does this, but I think it\nis very surprising that this pattern could lead to UAF.\n\nThe second problem is that things go wrong when aa_replace_current_label()\nruns with overridden credentials. aa_replace_current_label() bails out if\n`current_cred() != current_real_cred()` (mirroring the check in\nproc_pid_attr_write()), but this check can\u0027t actually reliably detect\noverridden credentials because the overridden creds can be the same as the\nobjective creds.\n\nSo in approximately the following scenario, things go wrong:\n\n1. task begins with \u003ccreds A\u003e (as both objective and subjective creds),\n   with refcount=2\n2. task grabs an extra reference on \u003ccreds A\u003e for overriding\n3. task calls override_creds(\u003ccreds A\u003e), which returns a pointer to the old\n   subjective creds (\u003ccreds A\u003e)\n4. task enters AppArmor LSM hook\n5. AppArmor checks that objective/subjective creds are equal\n6. AppArmor replaces both cred pointers with \u003ccreds B\u003e and drops 2 refs on\n   \u003ccreds A\u003e\n7. task leaves AppArmor LSM hook\n8. task calls revert_creds(\u003ccreds A\u003e)\n9. now task-\u003ecred is \u003ccreds A\u003e while task-\u003ereal_cred is \u003ccreds B\u003e, but the\n   task_struct logically holds two references to \u003ccreds B\u003e\n10. another task drops the extra reference on \u003ccreds A\u003e that was used for\n    overriding, refcount drops to 0\n11. now task-\u003ereal_cred points to freed creds\n\nAt this point, any access to current_cred() will be UAF.\n\nI have a test case where I run aa-disable on a profile while a process\nusing that profile is blocked on splice() from a FUSE passthrough file into\na full pipe; after the profile update, the pipe becomes empty, splice()\nresumes, the credentials go out of sync, and a subsequent getuid() syscall\nresults in a KASAN UAF splat.\n\nTo fix this, instead of directly replacing creds, do it via task_work that\nwill run at the end of the current syscall. (The point in time at which the\ncred replacement happens should have no correctness impact; it is just a\nperformance optimization to avoid unnecessarily touching the refcount of\nthe new label.)\n\nNote that AppArmor still performs direct cred replacements in the\nsb_pivotroot LSM hook after this change, and that direct cred replacements\ncan still happen in VFS -\u003ewrite() callbacks via proc_pid_attr_write().\n\nThere are two options for what to do with aa_dup_task_ctx(): Either\nexplicitly reset new-\u003elabel_replacement_pending after the entire\naa_task_ctx has been copied, or switch to manually copying members over.\nI am switching to manually copying members over because that should make\nbugs more obvious."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "baseScore": 7.8,
            "baseSeverity": "HIGH",
            "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
            "version": "3.1"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:L - begin_current_label_crit_section() runs from AppArmor LSM hooks on local VFS/socket syscalls after a stale profile replace. The confirmed UAF is splice from a FUSE passthrough file under override_creds() of the same cred object; unprivileged binfmt_misc register does the same around open_exec(). This is not a network receive path.\nAC:L - The attacker controls both sides: they replace the AppArmor profile to mark the task label FLAG_STALE, then invoke a syscall that override_creds()s with that same cred object (FUSE passthrough splice into a full pipe, or binfmt_misc OPEN_FILE register). The author\u2019s sequence is attacker-driven, not a victim-dependent race.\nPR:L - Making the label stale requires AppArmor policy replace (CAP_MAC_ADMIN), but unprivileged_userns_apparmor_policy defaults to 1 so userns root can load/replace policy. The LSM hooks and same-cred override_creds sites, including unprivileged binfmt_misc (FS_USERNS_MOUNT), are reachable without init-namespace root.\nUI:N - The attacker confines their own task, replaces their own policy, and triggers the override_creds VFS path themselves; no separate victim user action is required.\nS:U - Impact is a kernel struct cred use-after-free and local privilege escalation within the same kernel security authority. It is not a VM escape, IOMMU bypass, or cross-authority sandbox break.\nC:H - This is a use-after-free of struct cred: aa_replace_current_label() drops the task\u2019s refs during override_creds(), then revert_creds() restores a dangling cred pointer. Reclaiming the cred slab yields an arbitrary kernel read. Kernel UAF scores C:H.\nI:H - The same cred UAF is a classic heap-spray primitive on struct cred, enabling arbitrary writes and control-flow hijacking / privilege escalation. Kernel UAF scores I:H.\nA:H - The author reproduced a KASAN use-after-free of current_cred() on a later getuid() after the FUSE splice sequence; dereferencing the freed cred oopses or panics the kernel."
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-10-21T12:48:39.169Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/089f10b47843a86fa03a8c20799c9353660e5288"
        },
        {
          "url": "https://git.kernel.org/stable/c/361488984d668235438faac28635f3632351407f"
        },
        {
          "url": "https://git.kernel.org/stable/c/587a6a92b93ec314c583bbf747413af170d42540"
        },
        {
          "url": "https://git.kernel.org/stable/c/580f777d6d9fd07fc034bd1aeba5c30fd48871a2"
        },
        {
          "url": "https://git.kernel.org/stable/c/3f4ae5fab613dca01d6a2a8210dd832e009fcf47"
        }
      ],
      "title": "apparmor: fix cred UAF caused by begin_current_label_crit_section()",
      "x_generator": {
        "engine": "bippy-1.2.0"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2026-89762",
    "datePublished": "2026-09-11T19:47:03.601Z",
    "dateReserved": "2026-09-11T19:38:34.764Z",
    "dateUpdated": "2026-10-21T12:48:39.169Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}


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