ghsa-8g29-7ww6-62wf
Vulnerability from github
Published
2024-02-28 09:30
Modified
2024-11-04 18:31
Severity ?
7.7 (High) - CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H
Details

In the Linux kernel, the following vulnerability has been resolved:

sched/fair: Fix shift-out-of-bounds in load_balance()

Syzbot reported a handful of occurrences where an sd->nr_balance_failed can grow to much higher values than one would expect.

A successful load_balance() resets it to 0; a failed one increments it. Once it gets to sd->cache_nice_tries + 3, this should trigger an active balance, which will either set it to sd->cache_nice_tries+1 or reset it to 0. However, in case the to-be-active-balanced task is not allowed to run on env->dst_cpu, then the increment is done without any further modification.

This could then be repeated ad nauseam, and would explain the absurdly high values reported by syzbot (86, 149). VincentG noted there is value in letting sd->cache_nice_tries grow, so the shift itself should be fixed. That means preventing:

""" If the value of the right operand is negative or is greater than or equal to the width of the promoted left operand, the behavior is undefined. """

Thus we need to cap the shift exponent to BITS_PER_TYPE(typeof(lefthand)) - 1.

I had a look around for other similar cases via coccinelle:

@expr@ position pos; expression E1; expression E2; @@ ( E1 >> E2@pos | E1 >> E2@pos )

@cst depends on expr@ position pos; expression expr.E1; constant cst; @@ ( E1 >> cst@pos | E1 << cst@pos )

@script:python depends on !cst@ pos << expr.pos; exp << expr.E2; @@ # Dirty hack to ignore constexpr if exp.upper() != exp: coccilib.report.print_report(pos[0], "Possible UB shift here")

The only other match in kernel/sched is rq_clock_thermal() which employs sched_thermal_decay_shift, and that exponent is already capped to 10, so that one is fine.

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{
  "affected": [],
  "aliases": [
    "CVE-2021-47044"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-125"
    ],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2024-02-28T09:15:40Z",
    "severity": "HIGH"
  },
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nsched/fair: Fix shift-out-of-bounds in load_balance()\n\nSyzbot reported a handful of occurrences where an sd-\u003enr_balance_failed can\ngrow to much higher values than one would expect.\n\nA successful load_balance() resets it to 0; a failed one increments\nit. Once it gets to sd-\u003ecache_nice_tries + 3, this *should* trigger an\nactive balance, which will either set it to sd-\u003ecache_nice_tries+1 or reset\nit to 0. However, in case the to-be-active-balanced task is not allowed to\nrun on env-\u003edst_cpu, then the increment is done without any further\nmodification.\n\nThis could then be repeated ad nauseam, and would explain the absurdly high\nvalues reported by syzbot (86, 149). VincentG noted there is value in\nletting sd-\u003ecache_nice_tries grow, so the shift itself should be\nfixed. That means preventing:\n\n  \"\"\"\n  If the value of the right operand is negative or is greater than or equal\n  to the width of the promoted left operand, the behavior is undefined.\n  \"\"\"\n\nThus we need to cap the shift exponent to\n  BITS_PER_TYPE(typeof(lefthand)) - 1.\n\nI had a look around for other similar cases via coccinelle:\n\n  @expr@\n  position pos;\n  expression E1;\n  expression E2;\n  @@\n  (\n  E1 \u003e\u003e E2@pos\n  |\n  E1 \u003e\u003e E2@pos\n  )\n\n  @cst depends on expr@\n  position pos;\n  expression expr.E1;\n  constant cst;\n  @@\n  (\n  E1 \u003e\u003e cst@pos\n  |\n  E1 \u003c\u003c cst@pos\n  )\n\n  @script:python depends on !cst@\n  pos \u003c\u003c expr.pos;\n  exp \u003c\u003c expr.E2;\n  @@\n  # Dirty hack to ignore constexpr\n  if exp.upper() != exp:\n     coccilib.report.print_report(pos[0], \"Possible UB shift here\")\n\nThe only other match in kernel/sched is rq_clock_thermal() which employs\nsched_thermal_decay_shift, and that exponent is already capped to 10, so\nthat one is fine.",
  "id": "GHSA-8g29-7ww6-62wf",
  "modified": "2024-11-04T18:31:16Z",
  "published": "2024-02-28T09:30:38Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2021-47044"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/2f3eab368e313dba35fc2f51ede778bf7b030b54"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/39a2a6eb5c9b66ea7c8055026303b3aa681b49a5"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/805cea93e66ca7deaaf6ad3b67224ce47c104c2f"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/80862cbf76c2646f709a57c4517aefe0b094c774"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H",
      "type": "CVSS_V3"
    }
  ]
}


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