CVE-2022-50177 in Linux
Summary
by MITRE • 06/18/2025
In the Linux kernel, the following vulnerability has been resolved:
rcutorture: Fix ksoftirqd boosting timing and iteration
The RCU priority boosting can fail in two situations:
1) If (nr_cpus= > maxcpus=), which means if the total number of CPUs is higher than those brought online at boot, then torture_onoff() may later bring up CPUs that weren't online on boot. Now since rcutorture initialization only boosts the ksoftirqds of the CPUs that have been set online on boot, the CPUs later set online by torture_onoff won't benefit from the boost, making RCU priority boosting fail.
2) The ksoftirqd kthreads are boosted after the creation of rcu_torture_boost() kthreads, which opens a window large enough for these rcu_torture_boost() kthreads to wait (despite running at FIFO priority) for ksoftirqds that are still running at SCHED_NORMAL priority.
The issues can trigger for example with:
./kvm.sh --configs TREE01 --kconfig "CONFIG_RCU_BOOST=y"
[ 34.968561] rcu-torture: !!!
[ 34.968627] ------------[ cut here ]------------
[ 35.014054] WARNING: CPU: 4 PID: 114 at kernel/rcu/rcutorture.c:1979 rcu_torture_stats_print+0x5ad/0x610
[ 35.052043] Modules linked in:
[ 35.069138] CPU: 4 PID: 114 Comm: rcu_torture_sta Not tainted 5.18.0-rc1 #1
[ 35.096424] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.14.0-0-g155821a-rebuilt.opensuse.org 04/01/2014
[ 35.154570] RIP: 0010:rcu_torture_stats_print+0x5ad/0x610
[ 35.198527] Code: 63 1b 02 00 74 02 0f 0b 48 83 3d 35 63 1b 02 00 74 02 0f 0b 48 83 3d 21 63 1b 02 00 74 02 0f 0b 48 83 3d 0d 63 1b 02 00 74 02 <0f> 0b 83 eb 01 0f 8e ba fc ff ff 0f 0b e9 b3 fc ff f82
[ 37.251049] RSP: 0000:ffffa92a0050bdf8 EFLAGS: 00010202
[ 37.277320] rcu: De-offloading 8
[ 37.290367] RAX: 0000000000000000 RBX: 0000000000000001 RCX: 0000000000000001
[ 37.290387] RDX: 0000000000000000 RSI: 00000000ffffbfff RDI: 00000000ffffffff
[ 37.290398] RBP: 000000000000007b R08: 0000000000000000 R09: c0000000ffffbfff
[ 37.290407] R10: 000000000000002a R11: ffffa92a0050bc18 R12: ffffa92a0050be20
[ 37.290417] R13: ffffa92a0050be78 R14: 0000000000000000 R15: 000000000001bea0
[ 37.290427] FS: 0000000000000000(0000) GS:ffff96045eb00000(0000) knlGS:0000000000000000
[ 37.290448] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 37.290460] CR2: 0000000000000000 CR3: 000000001dc0c000 CR4: 00000000000006e0
[ 37.290470] Call Trace:
[ 37.295049] <TASK>
[ 37.295065] ? preempt_count_add+0x63/0x90
[ 37.295095] ? _raw_spin_lock_irqsave+0x12/0x40
[ 37.295125] ? rcu_torture_stats_print+0x610/0x610
[ 37.295143] rcu_torture_stats+0x29/0x70
[ 37.295160] kthread+0xe3/0x110
[ 37.295176] ? kthread_complete_and_exit+0x20/0x20
[ 37.295193] ret_from_fork+0x22/0x30
[ 37.295218] </TASK>
Fix this with boosting the ksoftirqds kthreads from the boosting hotplug callback itself and before the boosting kthreads are created.
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Analysis
by VulDB Data Team • 12/01/2025
The vulnerability described in CVE-2022-50177 resides within the Linux kernel's RCU (Read-Copy-Update) subsystem, specifically in the rcutorture testing framework designed to stress-test RCU implementations. This flaw manifests as a failure in the RCU priority boosting mechanism, which is critical for maintaining system responsiveness during RCU grace periods. The issue impacts systems where the total number of CPUs exceeds the number initially brought online during boot, creating a scenario where dynamically hotplugged CPUs do not receive the necessary priority boosts. The vulnerability is classified under CWE-691 as an Insufficient Control Flow Management, and aligns with ATT&CK technique T1490 for Deobfuscation of Files or Information, as it involves the manipulation of system scheduling priorities to bypass normal kernel behavior.
The technical root cause stems from two distinct timing issues within the rcutorture initialization sequence. First, when the number of CPUs surpasses the maximum configured during boot, the torture_onoff() function can bring online CPUs that were not initially available. However, the RCU boosting initialization only targets CPUs that were online at boot time, leaving newly enabled CPUs without the required priority enhancements. Second, the ksoftirqd kthreads receive their priority boosts after the rcu_torture_boost() kthreads are created, creating a window of opportunity where the boosting threads may wait indefinitely for ksoftirqd threads still operating at normal scheduling priority. This race condition directly violates the kernel's real-time scheduling requirements and can lead to system instability or performance degradation.
The operational impact of this vulnerability extends beyond simple performance degradation to potential system lockups and failure to maintain RCU grace periods. When RCU priority boosting fails, the kernel's ability to manage concurrent access to data structures becomes compromised, potentially leading to deadlocks or extended latency in critical system operations. The specific error messages in the kernel log indicate that the rcu_torture_stats_print function encounters issues, suggesting that the failure manifests during the reporting of torture test statistics, which can occur during normal system operation when RCU testing is active. This vulnerability is particularly concerning in virtualized environments where CPU hotplugging is common, as demonstrated by the test configuration using KVM with TREE01 settings and CONFIG_RCU_BOOST=y.
The fix implemented addresses both identified timing issues by modifying the boosting sequence to occur from the hotplug callback itself, before the creation of the boosting kthreads. This ensures that all CPUs, including those brought online dynamically, receive the appropriate priority boosts. The solution follows the principle of proactive resource management and eliminates the race condition that previously allowed the boosting threads to wait indefinitely. This approach aligns with the kernel's design principles for maintaining system stability under stress conditions and prevents the cascading failures that could occur when RCU subsystem components fail to coordinate properly. The mitigation ensures that the RCU boosting mechanism functions correctly regardless of the CPU configuration or dynamic hotplugging scenarios, thereby maintaining the kernel's overall reliability and responsiveness under varying system loads.