CVE-2022-49540: Vulnerability in Linux Linux

Severity: mediumType: vulnerabilityCVE-2022-49540

In the Linux kernel, the following vulnerability has been resolved: rcu-tasks: Fix race in schedule and flush work While booting secondary CPUs, cpus_read_[lock/unlock] is not keeping online cpumask stable. The transient online mask results in below calltrace. [ 0.324121] CPU1: Booted secondary processor 0x0000000001 [0x410fd083] [ 0.346652] Detected PIPT I-cache on CPU2 [ 0.347212] CPU2: Booted secondary processor 0x0000000002 [0x410fd083] [ 0.377255] Detected PIPT I-cache on CPU3 [ 0.377823] CPU3: Booted secondary processor 0x0000000003 [0x410fd083] [ 0.379040] ------------[ cut here ]------------ [ 0.383662] WARNING: CPU: 0 PID: 10 at kernel/workqueue.c:3084 __flush_work+0x12c/0x138 [ 0.384850] Modules linked in: [ 0.385403] CPU: 0 PID: 10 Comm: rcu_tasks_rude_ Not tainted 5.17.0-rc3-v8+ #13 [ 0.386473] Hardware name: Raspberry Pi 4 Model B Rev 1.4 (DT) [ 0.387289] pstate: 20000005 (nzCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--) [ 0.388308] pc : __flush_work+0x12c/0x138 [ 0.388970] lr : __flush_work+0x80/0x138 [ 0.389620] sp : ffffffc00aaf3c60 [ 0.390139] x29: ffffffc00aaf3d20 x28: ffffffc009c16af0 x27: ffffff80f761df48 [ 0.391316] x26: 0000000000000004 x25: 0000000000000003 x24: 0000000000000100 [ 0.392493] x23: ffffffffffffffff x22: ffffffc009c16b10 x21: ffffffc009c16b28 [ 0.393668] x20: ffffffc009e53861 x19: ffffff80f77fbf40 x18: 00000000d744fcc9 [ 0.394842] x17: 000000000000000b x16: 00000000000001c2 x15: ffffffc009e57550 [ 0.396016] x14: 0000000000000000 x13: ffffffffffffffff x12: 0000000100000000 [ 0.397190] x11: 0000000000000462 x10: ffffff8040258008 x9 : 0000000100000000 [ 0.398364] x8 : 0000000000000000 x7 : ffffffc0093c8bf4 x6 : 0000000000000000 [ 0.399538] x5 : 0000000000000000 x4 : ffffffc00a976e40 x3 : ffffffc00810444c [ 0.400711] x2 : 0000000000000004 x1 : 0000000000000000 x0 : 0000000000000000 [ 0.401886] Call trace: [ 0.402309] __flush_work+0x12c/0x138 [ 0.402941] schedule_on_each_cpu+0x228/0x278 [ 0.403693] rcu_tasks_rude_wait_gp+0x130/0x144 [ 0.404502] rcu_tasks_kthread+0x220/0x254 [ 0.405264] kthread+0x174/0x1ac [ 0.405837] ret_from_fork+0x10/0x20 [ 0.406456] irq event stamp: 102 [ 0.406966] hardirqs last enabled at (101): [<ffffffc0093c8468>] _raw_spin_unlock_irq+0x78/0xb4 [ 0.408304] hardirqs last disabled at (102): [<ffffffc0093b8270>] el1_dbg+0x24/0x5c [ 0.409410] softirqs last enabled at (54): [<ffffffc0081b80c8>] local_bh_enable+0xc/0x2c [ 0.410645] softirqs last disabled at (50): [<ffffffc0081b809c>] local_bh_disable+0xc/0x2c [ 0.411890] ---[ end trace 0000000000000000 ]--- [ 0.413000] smp: Brought up 1 node, 4 CPUs [ 0.413762] SMP: Total of 4 processors activated. [ 0.414566] CPU features: detected: 32-bit EL0 Support [ 0.415414] CPU features: detected: 32-bit EL1 Support [ 0.416278] CPU features: detected: CRC32 instructions [ 0.447021] Callback from call_rcu_tasks_rude() invoked. [ 0.506693] Callback from call_rcu_tasks() invoked. This commit therefore fixes this issue by applying a single-CPU optimization to the RCU Tasks Rude grace-period process. The key point here is that the purpose of this RCU flavor is to force a schedule on each online CPU since some past event. But the rcu_tasks_rude_wait_gp() function runs in the context of the RCU Tasks Rude's grace-period kthread, so there must already have been a context switch on the current CPU since the call to either synchronize_rcu_tasks_rude() or call_rcu_tasks_rude(). So if there is only a single CPU online, RCU Tasks Rude's grace-period kthread does not need to anything at all. It turns out that the rcu_tasks_rude_wait_gp() function's call to schedule_on_each_cpu() causes problems during early boot. During that time, there is only one online CPU, namely the boot CPU. Therefore, applying this single-CPU optimization fixes early-boot instances of this problem.

AI Analysis

Technical Summary

CVE-2022-49540 is a vulnerability in the Linux kernel related to the Read-Copy-Update (RCU) subsystem, specifically within the RCU Tasks Rude grace-period mechanism. The issue arises during the early boot process of secondary CPUs, where the online CPU mask is transient and unstable due to improper locking of cpus_read_lock/unlock. This instability leads to a race condition causing kernel warnings and potential system instability or crashes. The root cause is that the rcu_tasks_rude_wait_gp() function calls schedule_on_each_cpu(), which attempts to schedule work on all online CPUs. However, during early boot, only the primary CPU is online, and this call leads to a problematic race condition. The fix involves applying a single-CPU optimization that bypasses unnecessary scheduling when only one CPU is online, preventing the race and ensuring stable booting of secondary CPUs. This vulnerability affects Linux kernel versions around 5.17.0-rc3 and likely other versions with similar RCU task handling code. The issue is primarily a kernel-level race condition that can cause kernel warnings and potential instability during CPU initialization, particularly on ARM-based platforms like Raspberry Pi 4, as indicated in the logs. No known exploits are reported in the wild, and the vulnerability requires conditions present only during early CPU boot, limiting its exploitation scope.

Potential Impact

For European organizations, the impact of CVE-2022-49540 is primarily on systems running affected Linux kernel versions, especially those deployed on ARM architectures or embedded devices such as Raspberry Pi 4 or similar hardware used in industrial control, IoT, or edge computing environments. The vulnerability can cause kernel instability or crashes during system boot, potentially leading to denial of service (DoS) conditions. While it does not directly lead to privilege escalation or data breaches, the instability could disrupt critical services or automated systems relying on affected Linux kernels. Organizations using Linux-based infrastructure for servers, network devices, or embedded systems should be aware of potential boot-time reliability issues. The limited attack surface and requirement for early boot conditions reduce the risk of remote exploitation, but local attackers or faulty hardware initialization could trigger the issue. Systems that require high availability or operate in sensitive environments may experience operational disruptions if not patched.

Mitigation Recommendations

1. Update the Linux kernel to the latest stable version that includes the fix for CVE-2022-49540. Kernel maintainers have applied a single-CPU optimization to the RCU Tasks Rude grace-period code to prevent the race condition. 2. For embedded or ARM-based devices (e.g., Raspberry Pi 4), ensure firmware and kernel updates are applied promptly, as these devices are more likely to encounter this issue. 3. In environments where kernel updates are delayed, consider monitoring kernel logs for warnings related to __flush_work and RCU tasks during boot to detect potential issues early. 4. Test kernel updates in staging environments to verify stable boot sequences, especially on multi-core ARM systems. 5. Avoid custom kernel patches or configurations that alter RCU task scheduling without thorough testing, as these may reintroduce the race condition. 6. Implement robust system monitoring and automated reboot mechanisms to recover from potential boot-time failures caused by this vulnerability. 7. Engage with Linux distribution vendors to ensure timely patch deployment and backporting for long-term support kernels used in production.