CVE-2021-47553 sched/scs: Reset task stack state in bringup_cpu()

What is CVE-2021-47553?

CVE-2021-47553 is a high-severity memory corruption vulnerability in Linux kernel, published on 24 May 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/scs: Reset task stack state in bringup_cpu()

To hot unplug a CPU, the idle task on that CPU calls a few layers of C code before finally leaving the kernel. When KASAN is in use, poisoned shadow is left around for each of the active stack frames, and when shadow call stacks are in use. When shadow call stacks (SCS) are in use the task's saved SCS SP is left pointing at an arbitrary point within the task's shadow call stack.

When a CPU is offlined than onlined back into the kernel, this stale state can adversely affect execution. Stale KASAN shadow can alias new stackframes and result in bogus KASAN warnings. A stale SCS SP is effectively a memory leak, and prevents a portion of the shadow call stack being used. Across a number of hotplug cycles the idle task's entire shadow call stack can become unusable.

We previously fixed the KASAN issue in commit:

e1b77c92981a5222 ("sched/kasan: remove stale KASAN poison after hotplug")

... by removing any stale KASAN stack poison immediately prior to onlining a CPU.

Subsequently in commit:

f1a0a376ca0c4ef1 ("sched/core: Initialize the idle task with preemption disabled")

... the refactoring left the KASAN and SCS cleanup in one-time idle thread initialization code rather than something invoked prior to each CPU being onlined, breaking both as above.

We fixed SCS (but not KASAN) in commit:

63acd42c0d4942f7 ("sched/scs: Reset the shadow stack when idle_task_exit")

... but as this runs in the context of the idle task being offlined it's potentially fragile.

To fix these consistently and more robustly, reset the SCS SP and KASAN shadow of a CPU's idle task immediately before we online that CPU in bringup_cpu(). This ensures the idle task always has a consistent state when it is running, and removes the need to so so when exiting an idle task.

Whenever any thread is created, dup_task_struct() will give the task a stack which is free of KASAN shadow, and initialize the task's SCS SP, so there's no need to specially initialize either for idle thread within init_idle(), as this was only necessary to handle hotplug cycles.

I've tested this on arm64 with:

* gcc 11.1.0, defconfig +KASAN_INLINE, KASAN_STACK * clang 12.0.0, defconfig +KASAN_INLINE, KASAN_STACK, SHADOW_CALL_STACK

... offlining and onlining CPUS with:

| while true; do | for C in /sys/devices/system/cpu/cpu*/online; do | echo 0 > $C; | echo 1 > $C; | done | done

Weakness: CWE-908

Which versions are affected by CVE-2021-47553?

It affects 1 product.

Products include Linux kernel.

How do I fix CVE-2021-47553?

Apply the vendor's patch.

Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.

Is CVE-2021-47553 being exploited?

There is no known exploitation of CVE-2021-47553 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
• How this page is made

Also known as DEBIAN-CVE-2021-47553, UBUNTU-CVE-2021-47553.