CVE-2026-63799 (GCVE-0-2026-63799)

Vulnerability from cvelistv5 – Published: 2026-07-19 12:02 – Updated: 2026-07-20 13:40
VLAI
Title
sched/mmcid: Fix OOB clear_bit when CID is MM_CID_UNSET in fixup path
Summary
In the Linux kernel, the following vulnerability has been resolved: sched/mmcid: Fix OOB clear_bit when CID is MM_CID_UNSET in fixup path In mm_cid_fixup_cpus_to_tasks(), when rq->curr has the target mm and mm_cid.active is set, the CID is checked with cid_in_transit() before setting the transition bit. In per-CPU mode a newly forked or exec'd task can be running with mm_cid.cid == MM_CID_UNSET because CIDs are assigned lazily on schedule-in. With cid_in_transit() the guard passes for MM_CID_UNSET (no transit bit), converts it to MM_CID_UNSET | MM_CID_TRANSIT and stores it back; later mm_cid_schedout() feeds this to clear_bit() with MM_CID_UNSET as the bit number, triggering an out-of-bounds write. Symptoms: this is genuine memory corruption, but a bounded out-of-bounds write, not an arbitrary one. MM_CID_UNSET is the fixed sentinel BIT(31), so once the bad value reaches mm_cid_schedout() the cid_from_transit_cid() strip leaves MM_CID_UNSET, which fails the "cid < max_cids" convergence test and falls into mm_drop_cid() -> clear_bit(MM_CID_UNSET, mm_cidmask(mm)). The cid bitmap is embedded in the mm_struct slab object (after cpu_bitmap and mm_cpus_allowed) and is only num_possible_cpus() bits wide, so clearing bit 31 is a deterministic OOB bit-clear at a fixed offset of 2^31 / 8 == 256 MiB past the bitmap base. The address is not attacker-influenced (fixed sentinel -> fixed offset) and the op only clears a single bit; what sits 256 MiB further along the direct map is whatever kernel object happens to live there, so this corrupts one bit of unpredictable kernel memory -- it is not an arbitrary-address or arbitrary-value write. It triggers only in per-CPU CID mode, when a CPU is running an active task of the target mm whose cid is still MM_CID_UNSET -- the fork()/execve() window before that task's next schedule-in assigns it a real CID -- and a per-CPU -> per-task fixup walks over it (the mode fallback driven by a thread exit, sched_mm_cid_exit(), or by the deferred max_cids recompute in mm_cid_work_fn()). In practice syzkaller surfaced it as a KASAN use-after-free reported in __schedule -> mm_cid_switch_to, where the offending clear_bit() is inlined via mm_cid_schedout() -> mm_drop_cid(). Guard the transition-bit assignment against MM_CID_UNSET, in addition to the existing cid_in_transit() check, so the bit is only set on a genuine task-owned CID. A CPU-owned (MM_CID_ONCPU) CID of a running active task is handled by the cid_on_cpu(pcp->cid) branch above and never reaches this path, so excluding MM_CID_UNSET (and the already-transitioning case) is sufficient.
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
Linux Linux Affected: fbd0e71dc370af73f6b316e4de9eed273dd90340 , < 8d32856fb72ba976d9c87ba405fd17e80419934c (git)
Affected: fbd0e71dc370af73f6b316e4de9eed273dd90340 , < de3ab9bd3133899efb92e4cd05ba4203e58fc0a3 (git)
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Linux Linux Affected: 6.19
Unaffected: 0 , < 6.19 (semver)
Unaffected: 7.1.3 , ≤ 7.1.* (semver)
Unaffected: 7.2-rc1 , ≤ * (original_commit_for_fix)
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{
  "containers": {
    "cna": {
      "affected": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "kernel/sched/core.c"
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          "vendor": "Linux",
          "versions": [
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              "status": "affected",
              "version": "fbd0e71dc370af73f6b316e4de9eed273dd90340",
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          "vendor": "Linux",
          "versions": [
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              "cpeMatch": [
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                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "7.1.3",
                  "versionStartIncluding": "6.19",
                  "vulnerable": true
                },
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "7.2-rc1",
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      "descriptions": [
        {
          "lang": "en",
          "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-\u003ecurr 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\u0027d\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 \u003c max_cids\" convergence\ntest and falls into mm_drop_cid() -\u003e 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 -\u003e 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\u0027s next schedule-in assigns it a\nreal CID -- and a per-CPU -\u003e 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 -\u003e mm_cid_switch_to, where the offending clear_bit() is inlined\nvia mm_cid_schedout() -\u003e 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-\u003ecid) branch above and never reaches\nthis path, so excluding MM_CID_UNSET (and the already-transitioning case)\nis sufficient."
        }
      ],
      "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"
          }
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      "providerMetadata": {
        "dateUpdated": "2026-07-20T13:40:09.516Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
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      "references": [
        {
          "url": "https://git.kernel.org/stable/c/8d32856fb72ba976d9c87ba405fd17e80419934c"
        },
        {
          "url": "https://git.kernel.org/stable/c/de3ab9bd3133899efb92e4cd05ba4203e58fc0a3"
        }
      ],
      "title": "sched/mmcid: Fix OOB clear_bit when CID is MM_CID_UNSET in fixup path",
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    "assignerShortName": "Linux",
    "cveId": "CVE-2026-63799",
    "datePublished": "2026-07-19T12:02:06.341Z",
    "dateReserved": "2026-07-19T07:54:57.013Z",
    "dateUpdated": "2026-07-20T13:40:09.516Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2",
  "vulnerability-lookup:meta": {
    "epss": {
      "cve": "CVE-2026-63799",
      "date": "2026-07-21",
      "epss": "0.00121",
      "percentile": "0.02261"
    },
    "nvd": "{\"cve\":{\"id\":\"CVE-2026-63799\",\"sourceIdentifier\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"published\":\"2026-07-19T12:16:52.433\",\"lastModified\":\"2026-07-20T15:16:46.587\",\"vulnStatus\":\"Received\",\"cveTags\":[],\"descriptions\":[{\"lang\":\"en\",\"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-\u003ecurr 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\u0027d\\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 \u003c max_cids\\\" convergence\\ntest and falls into mm_drop_cid() -\u003e 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 -\u003e 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\u0027s next schedule-in assigns it a\\nreal CID -- and a per-CPU -\u003e 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 -\u003e mm_cid_switch_to, where the offending clear_bit() is inlined\\nvia mm_cid_schedout() -\u003e 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-\u003ecid) branch above and never reaches\\nthis path, so excluding MM_CID_UNSET (and the already-transitioning case)\\nis sufficient.\"}],\"affected\":[{\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"affectedData\":[{\"vendor\":\"Linux\",\"product\":\"Linux\",\"defaultStatus\":\"unaffected\",\"programFiles\":[\"kernel/sched/core.c\"],\"repo\":\"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\",\"versions\":[{\"version\":\"fbd0e71dc370af73f6b316e4de9eed273dd90340\",\"lessThan\":\"8d32856fb72ba976d9c87ba405fd17e80419934c\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"fbd0e71dc370af73f6b316e4de9eed273dd90340\",\"lessThan\":\"de3ab9bd3133899efb92e4cd05ba4203e58fc0a3\",\"versionType\":\"git\",\"status\":\"affected\"}]},{\"vendor\":\"Linux\",\"product\":\"Linux\",\"defaultStatus\":\"affected\",\"programFiles\":[\"kernel/sched/core.c\"],\"repo\":\"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\",\"versions\":[{\"version\":\"6.19\",\"status\":\"affected\"},{\"version\":\"0\",\"lessThan\":\"6.19\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"7.1.3\",\"lessThanOrEqual\":\"7.1.*\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"7.2-rc1\",\"lessThanOrEqual\":\"*\",\"versionType\":\"original_commit_for_fix\",\"status\":\"unaffected\"}]}]}],\"metrics\":{\"cvssMetricV31\":[{\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"type\":\"Secondary\",\"cvssData\":{\"version\":\"3.1\",\"vectorString\":\"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H\",\"baseScore\":7.8,\"baseSeverity\":\"HIGH\",\"attackVector\":\"LOCAL\",\"attackComplexity\":\"LOW\",\"privilegesRequired\":\"LOW\",\"userInteraction\":\"NONE\",\"scope\":\"UNCHANGED\",\"confidentialityImpact\":\"HIGH\",\"integrityImpact\":\"HIGH\",\"availabilityImpact\":\"HIGH\"},\"exploitabilityScore\":1.8,\"impactScore\":5.9}]},\"references\":[{\"url\":\"https://git.kernel.org/stable/c/8d32856fb72ba976d9c87ba405fd17e80419934c\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"},{\"url\":\"https://git.kernel.org/stable/c/de3ab9bd3133899efb92e4cd05ba4203e58fc0a3\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"}]}}",
    "redhat_vex": {
      "aggregate_severity": "None",
      "current_release_date": "2026-07-21T08:04:31+00:00",
      "cve": "CVE-2026-63799",
      "id": "CVE-2026-63799",
      "initial_release_date": "2026-07-19T00:00:00+00:00",
      "product_status:known_not_affected": "274",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: sched/mmcid: Fix OOB clear_bit when CID is MM_CID_UNSET in fixup path",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-63799.json",
      "version": "3"
    }
  }
}



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Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

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  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
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  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


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