# CVE-2021-47044: linux: In the Linux kernel, the following vulnerability has been resolved:  sched/fair: Fix shift-out-of-bounds in load_bala...

CVE-2021-47044 is fixed in linux on 1 platform, first by DEBIAN-CVE-2021-47044, released February 28, 2024. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2021-47044
- Priority: High
- CVSS: 7.7 (high)
- EPSS: 0.0027
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Feb 28, 2024

## Description

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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 12.15 (bookworm) | 5.10.38-1 | [DEBIAN-CVE-2021-47044](https://patchblox.com/security/data/fix/linux/debian-cve-2021-47044.md) | Feb 28, 2024 |
| Linux | linux Debian 13.5 (trixie) | 5.10.38-1 | [DEBIAN-CVE-2021-47044](https://patchblox.com/security/data/fix/linux/debian-cve-2021-47044.md) | Feb 28, 2024 |
| Linux | linux Debian 14 | 5.10.38-1 | [DEBIAN-CVE-2021-47044](https://patchblox.com/security/data/fix/linux/debian-cve-2021-47044.md) | Feb 28, 2024 |

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