sched/mmcid: Fix OOB clear_bit when CID is MM_CID_UNSET in fixup path

{
  "id": "d4535dab10da73aa868ff183745bf0fa783e1c51601377408779afbf284c86cd",
  "url": "https://osv.dev/vulnerability/CVE-2026-63799",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "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 ",
    "evidence": "osv:CVE-2026-63799",
    "products": [
      {
        "name": {
          "value": "Kernel",
          "evidence": "/affected/1/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "7.1.3",
            "evidence": "/affected/1/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-63799",
      "aliases": [
        "CVE-2026-63799"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
              "type": "GIT",
              "events": [
                {
                  "introduced": "fbd0e71dc370af73f6b316e4de9eed273dd90340"
                },
                {
                  "fixed": "8d32856fb72ba976d9c87ba405fd17e80419934c"
                },
                {
                  "fixed": "de3ab9bd3133899efb92e4cd05ba4203e58fc0a3"
                }
              ]
            }
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-63799.json"
          }
        },
        {
          "ranges": [
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "6.19.0"
                },
                {
                  "fixed": "7.1.3"
                }
              ]
            }
          ],
          "package": {
            "name": "Kernel",
            "ecosystem": "Linux"
          },
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-63799.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:39.831292341Z",
      "provider": "osv",
      "references": [
        "https://git.kernel.org/stable/c/8d32856fb72ba976d9c87ba405fd17e80419934c",
        "https://git.kernel.org/stable/c/de3ab9bd3133899efb92e4cd05ba4203e58fc0a3",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/63xxx/CVE-2026-63799.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-63799",
        "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"
      ],
      "provider_metadata": {
        "cna_assigner": "Linux",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/63xxx/CVE-2026-63799.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-63799",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 506919,
  "published_at": "2026-07-19T12:02:06.341+00:00"
}