CVE-2021-47044

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.

Affected Software

VendorProductVersion RangeStatus
LinuxLinux5a7f555904671c0737819fe4d19bd6143de3f6c0 < 80862cbf76c2646f709a57c4517aefe0b094c774affected
LinuxLinux5a7f555904671c0737819fe4d19bd6143de3f6c0 < 2f3eab368e313dba35fc2f51ede778bf7b030b54affected
LinuxLinux5a7f555904671c0737819fe4d19bd6143de3f6c0 < 805cea93e66ca7deaaf6ad3b67224ce47c104c2faffected
LinuxLinux5a7f555904671c0737819fe4d19bd6143de3f6c0 < 39a2a6eb5c9b66ea7c8055026303b3aa681b49a5affected
LinuxLinux5.10affected
LinuxLinux0 < 5.10unaffected
LinuxLinux5.10.37 <= 5.10.*unaffected
LinuxLinux5.11.21 <= 5.11.*unaffected
LinuxLinux5.12.4 <= 5.12.*unaffected
LinuxLinux5.13 <= *unaffected

Weaknesses

ADP Enrichment

CISA ADP Vulnrichment

  • SSVC:
  • Exploitation: none
    • Automatable: no
    • Technical Impact: partial

CVE Program Container

Additional References

References