CVE-2021-47044: sched/fair: Fix shift-out-of-bounds in load_balance()
First published: Wed Feb 28 2024(Updated: )
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.
Credit: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
| Affected Software | Affected Version | How to fix |
|---|---|---|
| Linux Kernel | ||
| Linux Kernel | >=5.10<5.10.37 | |
| Linux Kernel | >=5.11<5.11.21 | |
| Linux Kernel | >=5.12<5.12.4 |
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Reference Links
Frequently Asked Questions
What is the severity of CVE-2021-47044?
CVE-2021-47044 has been assigned a low severity rating due to the nature of the vulnerability and its potential impact.
How do I fix CVE-2021-47044?
To fix CVE-2021-47044, ensure your Linux kernel is updated to the latest patched version that resolves this issue.
What does CVE-2021-47044 affect?
CVE-2021-47044 affects the Linux Kernel, specifically the load_balance() function within the scheduling subsystem.
Can CVE-2021-47044 be exploited remotely?
CVE-2021-47044 does not allow for remote exploitation as it requires local access to affect the kernel's operation.
What are the symptoms of CVE-2021-47044?
Symptoms of CVE-2021-47044 may include abnormal scheduling behavior in multi-threaded environments resulting in performance degradation.
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