CVE-2021-47044 sched/fair: Fix shift-out-of-bounds in load_balance()

What is CVE-2021-47044?

CVE-2021-47044 is a high-severity vulnerability in Linux kernel, published on 28 Feb 2024. It can only be exploited from the same machine or local network, with no action from the victim.

Official 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.

Weakness: CWE-125

Which versions are affected by CVE-2021-47044?

It affects 1 product.

Products include Linux kernel.

How do I fix CVE-2021-47044?

Apply the vendor's patch.

Is CVE-2021-47044 being exploited?

There is no known exploitation of CVE-2021-47044 in the wild and no public exploit code.

Other vulnerabilities in Linux kernel

• CVE-2026-53266 Linux Kernel Out-of-Bounds Write Exploited High 8.8
• CVE-2013-6282 Linux Kernel Improper Input Validation Exploited High 8.8
• CVE-2022-0185 Linux Kernel Heap-Based Buffer Overflow Exploited High 8.4
• CVE-2013-2094 Linux Kernel Privilege Escalation Exploited High 8.4
• CVE-2026-53362 Linux Kernel Unspecified Exploited High 7.8

All Linux kernel vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as DEBIAN-CVE-2021-47044, GHSA-8g29-7ww6-62wf, UBUNTU-CVE-2021-47044.