CVE-2021-47044

published 2024-02-28

CVE-2021-47044: 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…

Priority P3 35 high 7.7 CVSS 3.1

AV L AC L PR N UI N S U C H I N A H

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

13 ranges

Vendor	Product	Version range	Fixed in
debian	linux	< linux 5.10.38-1 (bookworm)	linux 5.10.38-1 (bookworm)
linux	linux	—	—
linux	linux	>= 5a7f555904671c0737819fe4d19bd6143de3f6c0 < 80862cbf76c2646f709a57c4517aefe0b094c774	80862cbf76c2646f709a57c4517aefe0b094c774
linux	linux	>= 5a7f555904671c0737819fe4d19bd6143de3f6c0 < 2f3eab368e313dba35fc2f51ede778bf7b030b54	2f3eab368e313dba35fc2f51ede778bf7b030b54
linux	linux	>= 5a7f555904671c0737819fe4d19bd6143de3f6c0 < 805cea93e66ca7deaaf6ad3b67224ce47c104c2f	805cea93e66ca7deaaf6ad3b67224ce47c104c2f
linux	linux	>= 5a7f555904671c0737819fe4d19bd6143de3f6c0 < 39a2a6eb5c9b66ea7c8055026303b3aa681b49a5	39a2a6eb5c9b66ea7c8055026303b3aa681b49a5
linux	linux_kernel	>= 0 < 5.10.38-1	5.10.38-1
linux	linux_kernel	>= 0 < 5.10.38-1	5.10.38-1
linux	linux_kernel	>= 0 < 5.10.38-1	5.10.38-1
linux	linux_kernel	>= 0 < 5.10.38-1	5.10.38-1
linux	linux_kernel	>= 5.10 < 5.10.37	5.10.37
linux	linux_kernel	>= 5.11 < 5.11.21	5.11.21
linux	linux_kernel	>= 5.12 < 5.12.4	5.12.4

CVSS provenance

nvd v3.1 7.7 HIGH CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H

osv 7.7 HIGH

vendor_debian 7.7 HIGH

vendor_redhat 7.7 HIGH

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