cve-2021-47044
Vulnerability from cvelistv5
Published
2024-02-28 08:13
Modified
2024-12-19 07:34
Severity ?
7.7 (High) - CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H
Summary
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.
References
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/2f3eab368e313dba35fc2f51ede778bf7b030b54
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/39a2a6eb5c9b66ea7c8055026303b3aa681b49a5
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/805cea93e66ca7deaaf6ad3b67224ce47c104c2f
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/80862cbf76c2646f709a57c4517aefe0b094c774
af854a3a-2127-422b-91ae-364da2661108https://git.kernel.org/stable/c/2f3eab368e313dba35fc2f51ede778bf7b030b54
af854a3a-2127-422b-91ae-364da2661108https://git.kernel.org/stable/c/39a2a6eb5c9b66ea7c8055026303b3aa681b49a5
af854a3a-2127-422b-91ae-364da2661108https://git.kernel.org/stable/c/805cea93e66ca7deaaf6ad3b67224ce47c104c2f
af854a3a-2127-422b-91ae-364da2661108https://git.kernel.org/stable/c/80862cbf76c2646f709a57c4517aefe0b094c774
Impacted products
Vendor Product Version
Linux Linux Version: 5.10
Show details on NVD website

To clipboard 

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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. 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Un load_balance() exitoso lo restablece a 0; uno fallido lo incrementa. Una vez que llegue a sd-\u0026gt;cache_nice_tries + 3, esto *deber\u00eda* activar un saldo activo, que lo establecer\u00e1 en sd-\u0026gt;cache_nice_tries+1 o lo restablecer\u00e1 a 0. Sin embargo, en caso de que el saldo est\u00e9 activo La tarea no puede ejecutarse en env-\u0026gt;dst_cpu, entonces el incremento se realiza sin ninguna modificaci\u00f3n adicional. Esto podr\u00eda repetirse hasta la saciedad y explicar\u00eda los valores absurdamente altos informados por syzbot (86, 149). VincentG se\u00f1al\u00f3 que es valioso dejar que sd-\u0026gt;cache_nice_tries crezca, por lo que el cambio en s\u00ed deber\u00eda corregirse. 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}


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