138 lines
9.4 KiB
JSON
138 lines
9.4 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-2026-63799",
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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": "2026-07-19T07:54:57.013Z",
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"datePublished": "2026-07-19T12:02:06.341Z",
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"dateUpdated": "2026-08-17T04:51:04.486Z"
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},
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"containers": {
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"cna": {
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"providerMetadata": {
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"orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
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"shortName": "Linux",
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"dateUpdated": "2026-08-17T04:51:04.486Z"
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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\nsched/mmcid: Fix OOB clear_bit when CID is MM_CID_UNSET in fixup path\n\nIn mm_cid_fixup_cpus_to_tasks(), when rq->curr has the target mm and\nmm_cid.active is set, the CID is checked with cid_in_transit() before\nsetting the transition bit. In per-CPU mode a newly forked or exec'd\ntask can be running with mm_cid.cid == MM_CID_UNSET because CIDs are\nassigned lazily on schedule-in. With cid_in_transit() the guard passes\nfor MM_CID_UNSET (no transit bit), converts it to MM_CID_UNSET |\nMM_CID_TRANSIT and stores it back; later mm_cid_schedout() feeds this\nto clear_bit() with MM_CID_UNSET as the bit number, triggering an\nout-of-bounds write.\n\nSymptoms: this is genuine memory corruption, but a bounded out-of-bounds\nwrite, not an arbitrary one. MM_CID_UNSET is the fixed sentinel BIT(31),\nso once the bad value reaches mm_cid_schedout() the cid_from_transit_cid()\nstrip leaves MM_CID_UNSET, which fails the \"cid < max_cids\" convergence\ntest and falls into mm_drop_cid() -> clear_bit(MM_CID_UNSET,\nmm_cidmask(mm)). The cid bitmap is embedded in the mm_struct slab object\n(after cpu_bitmap and mm_cpus_allowed) and is only num_possible_cpus()\nbits wide, so clearing bit 31 is a deterministic OOB bit-clear at a\nfixed offset of 2^31 / 8 == 256 MiB past the bitmap base. The address is\nnot attacker-influenced (fixed sentinel -> fixed offset) and the op only\nclears a single bit; what sits 256 MiB further along the direct map is\nwhatever kernel object happens to live there, so this corrupts one bit of\nunpredictable kernel memory -- it is not an arbitrary-address or\narbitrary-value write.\n\nIt triggers only in per-CPU CID mode, when a CPU is running an active\ntask of the target mm whose cid is still MM_CID_UNSET -- the\nfork()/execve() window before that task's next schedule-in assigns it a\nreal CID -- and a per-CPU -> per-task fixup walks over it (the mode\nfallback driven by a thread exit, sched_mm_cid_exit(), or by the deferred\nmax_cids recompute in mm_cid_work_fn()).\n\nIn practice syzkaller surfaced it as a KASAN use-after-free reported in\n__schedule -> mm_cid_switch_to, where the offending clear_bit() is inlined\nvia mm_cid_schedout() -> mm_drop_cid().\n\nGuard the transition-bit assignment against MM_CID_UNSET, in addition to\nthe existing cid_in_transit() check, so the bit is only set on a genuine\ntask-owned CID. A CPU-owned (MM_CID_ONCPU) CID of a running active task\nis handled by the cid_on_cpu(pcp->cid) branch above and never reaches\nthis path, so excluding MM_CID_UNSET (and the already-transitioning case)\nis sufficient."
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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 - The bug is reached only through local scheduler paths triggered by fork/clone, exit, and execve syscalls, not by any network, Bluetooth, or physical device interface.\nAC:L - An unprivileged attacker can deliberately create many threads to enter per-CPU CID mode, fork/exec a task that remains active with MM_CID_UNSET, and time thread exits or affinity-driven fixups to hit the vulnerable branch without depending on conditions outside their control.\nPR:L - Triggering the vulnerable fixup requires only standard unprivileged process operations (creating threads, forking, exiting) within one's own address space, including from unprivileged user namespaces.\nUI:N - Exploitation requires no action from any other user or administrator; the attacker drives the entire thread lifecycle and timing themselves.\nS:U - Impact is kernel memory corruption within the same kernel security boundary; it does not constitute a VM escape, sandbox breakout, or cross-authority boundary crossing.\nC:H - The out-of-bounds bit-clear corrupts unpredictable kernel memory at a fixed large offset, and any such memory corruption can potentially be leveraged for information disclosure even though the immediate primitive is a single-bit write.\nI:H - The vulnerability performs an out-of-bounds write via clear_bit() with bit number MM_CID_UNSET (BIT(31)), which is genuine kernel memory corruption in the integrity domain.\nA:H - Syzkaller reported this as a KASAN fault in __schedule -> mm_cid_switch_to, and corrupting arbitrary kernel memory can cause kernel oops, panic, or crash-level denial of service."
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}
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]
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}
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],
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"affected": [
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{
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"product": "Linux",
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"vendor": "Linux",
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"defaultStatus": "unaffected",
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"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
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"programFiles": [
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"kernel/sched/core.c"
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],
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"versions": [
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{
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"version": "fbd0e71dc370af73f6b316e4de9eed273dd90340",
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"lessThan": "8d32856fb72ba976d9c87ba405fd17e80419934c",
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"status": "affected",
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"versionType": "git"
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},
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{
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"version": "fbd0e71dc370af73f6b316e4de9eed273dd90340",
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"lessThan": "de3ab9bd3133899efb92e4cd05ba4203e58fc0a3",
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"status": "affected",
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"versionType": "git"
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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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"vendor": "Linux",
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"defaultStatus": "affected",
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"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
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"programFiles": [
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"kernel/sched/core.c"
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],
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"versions": [
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{
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"version": "6.19",
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"status": "affected"
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},
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{
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"version": "0",
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"lessThan": "6.19",
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"status": "unaffected",
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"versionType": "semver"
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},
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{
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"version": "7.1.3",
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"lessThanOrEqual": "7.1.*",
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"status": "unaffected",
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"versionType": "semver"
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},
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{
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"version": "7.2",
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"lessThanOrEqual": "*",
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"status": "unaffected",
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"versionType": "original_commit_for_fix"
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}
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]
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}
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],
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"cpeApplicability": [
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{
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"nodes": [
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{
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"operator": "OR",
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"negate": false,
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"cpeMatch": [
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{
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"vulnerable": true,
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"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
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"versionStartIncluding": "6.19",
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"versionEndExcluding": "7.1.3"
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},
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{
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"vulnerable": true,
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"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
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"versionStartIncluding": "6.19",
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"versionEndExcluding": "7.2"
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}
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]
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}
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]
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}
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],
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"references": [
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{
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"url": "https://git.kernel.org/stable/c/8d32856fb72ba976d9c87ba405fd17e80419934c"
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},
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{
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"url": "https://git.kernel.org/stable/c/de3ab9bd3133899efb92e4cd05ba4203e58fc0a3"
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}
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],
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"title": "sched/mmcid: Fix OOB clear_bit when CID is MM_CID_UNSET in fixup path",
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"x_generator": {
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