226 lines
15 KiB
JSON
226 lines
15 KiB
JSON
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"cveId": "CVE-2026-89762",
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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": "2026-09-11T19:38:34.764Z",
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"datePublished": "2026-09-11T19:47:03.601Z",
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"dateUpdated": "2026-10-30T12:32:59.196Z"
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"shortName": "Linux",
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"dateUpdated": "2026-10-30T12:32:59.196Z"
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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\napparmor: fix cred UAF caused by begin_current_label_crit_section()\n\nAppArmor's 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->euid;\n```\nI don't 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't 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 <creds A> (as both objective and subjective creds),\n with refcount=2\n2. task grabs an extra reference on <creds A> for overriding\n3. task calls override_creds(<creds A>), which returns a pointer to the old\n subjective creds (<creds A>)\n4. task enters AppArmor LSM hook\n5. AppArmor checks that objective/subjective creds are equal\n6. AppArmor replaces both cred pointers with <creds B> and drops 2 refs on\n <creds A>\n7. task leaves AppArmor LSM hook\n8. task calls revert_creds(<creds A>)\n9. now task->cred is <creds A> while task->real_cred is <creds B>, but the\n task_struct logically holds two references to <creds B>\n10. another task drops the extra reference on <creds A> that was used for\n overriding, refcount drops to 0\n11. now task->real_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 ->write() callbacks via proc_pid_attr_write().\n\nThere are two options for what to do with aa_dup_task_ctx(): Either\nexplicitly reset new->label_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."
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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:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
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"baseScore": 7.8,
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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: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’s 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’s 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."
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}
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