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Out-of-bounds Read The advisory has been revoked - it doesn't affect any version of package kernel-rt-modules-internal  (opens in a new tab)

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EPSS
0.05% (19th percentile)

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  • Snyk IDSNYK-CENTOS8-KERNELRTMODULESINTERNAL-6337855
  • published29 Feb 2024
  • disclosed28 Feb 2024
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Introduced: 28 Feb 2024

CVE-2021-47044  (opens in a new tab)
CWE-125  (opens in a new tab)

Amendment

The Centos security team deemed this advisory irrelevant for Centos:8.

NVD Description

Note: Versions mentioned in the description apply only to the upstream kernel-rt-modules-internal package and not the kernel-rt-modules-internal package as distributed by Centos.

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.