CVE-2022-49181 in Linux
Summary
by MITRE • 02/26/2025
In the Linux kernel, the following vulnerability has been resolved:
xen: fix is_xen_pmu()
is_xen_pmu() is taking the cpu number as parameter, but it is not using it. Instead it just tests whether the Xen PMU initialization on the current cpu did succeed. As this test is done by checking a percpu pointer, preemption needs to be disabled in order to avoid switching the cpu while doing the test. While resuming from suspend() this seems not to be the case:
[ 88.082751] ACPI: PM: Low-level resume complete
[ 88.087933] ACPI: EC: EC started
[ 88.091464] ACPI: PM: Restoring platform NVS memory
[ 88.097166] xen_acpi_processor: Uploading Xen processor PM info
[ 88.103850] Enabling non-boot CPUs ...
[ 88.108128] installing Xen timer for CPU 1
[ 88.112763] BUG: using smp_processor_id() in preemptible [00000000] code: systemd-sleep/7138
[ 88.122256] caller is is_xen_pmu+0x12/0x30
[ 88.126937] CPU: 0 PID: 7138 Comm: systemd-sleep Tainted: G W 5.16.13-2.fc32.qubes.x86_64 #1
[ 88.137939] Hardware name: Star Labs StarBook/StarBook, BIOS 7.97 03/21/2022
[ 88.145930] Call Trace:
[ 88.148757]
[ 88.151193] dump_stack_lvl+0x48/0x5e
[ 88.155381] check_preemption_disabled+0xde/0xe0
[ 88.160641] is_xen_pmu+0x12/0x30
[ 88.164441] xen_smp_intr_init_pv+0x75/0x100
Fix that by replacing is_xen_pmu() by a simple boolean variable which reflects the Xen PMU initialization state on cpu 0.
Modify xen_pmu_init() to return early in case it is being called for a cpu other than cpu 0 and the boolean variable not being set.
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Analysis
by VulDB Data Team • 02/26/2025
The vulnerability described in CVE-2022-49181 represents a critical preemption safety issue within the Linux kernel's Xen hypervisor support subsystem. This flaw manifests specifically in the is_xen_pmu() function which is designed to check whether Xen Performance Monitoring Unit (PMU) initialization has succeeded on a particular CPU. The function accepts a CPU number as a parameter but fails to utilize this information properly, instead relying on per-CPU pointer checks that require disabling preemption to prevent CPU migration during execution. This design creates a dangerous condition where the function attempts to access per-CPU data structures while preemption is enabled, leading to potential kernel oops and system instability.
The technical implementation flaw stems from the function's improper handling of CPU context switching during system resume operations from suspend states. When the system resumes from suspend, the kernel's preemption mechanism is not properly disabled during the critical section where is_xen_pmu() performs its checks, resulting in a direct violation of kernel safety requirements. The error message clearly indicates that smp_processor_id() is being called from preemptible code, which violates kernel design principles and triggers the kernel's built-in safety mechanisms that prevent such operations. This specific violation occurs during the xen_smp_intr_init_pv() function call sequence, demonstrating how the vulnerability propagates through the kernel's interrupt handling and CPU initialization pathways.
The operational impact of this vulnerability extends beyond simple system instability to potentially compromising the entire system's reliability during power management operations. The issue specifically manifests during system resume from suspend states, where the kernel attempts to reinitialize CPU resources including Xen PMU support. This creates a race condition scenario where the kernel's assumption about CPU context stability is violated, potentially leading to kernel panics, system crashes, or unpredictable behavior during critical power management operations. The vulnerability affects systems running Linux kernels with Xen hypervisor support, particularly those implementing Qubes OS or similar virtualization environments that rely on Xen PMU functionality for performance monitoring and power management.
The fix implemented addresses this vulnerability by replacing the problematic is_xen_pmu() function with a simple boolean variable that tracks the Xen PMU initialization state specifically on CPU 0, eliminating the need for per-CPU pointer checks during preemption-sensitive operations. This approach directly addresses the core issue identified in the Linux kernel's design by ensuring that CPU context checks are performed in a safe, non-preemptible environment. The modification to xen_pmu_init() ensures that the function returns early when called for CPUs other than CPU 0, preventing the unsafe per-CPU pointer access patterns that caused the original vulnerability. This remediation aligns with established kernel security practices and addresses the underlying CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization) vulnerability pattern by ensuring proper synchronization and context handling during critical kernel operations.
The vulnerability's classification under the ATT&CK framework would relate to privilege escalation and system stability compromise through kernel-level modifications. The fix demonstrates proper kernel security engineering practices by eliminating unsafe direct access to per-CPU data structures during preemption-sensitive code sections, which aligns with the principle of minimizing kernel code execution contexts where preemption is enabled. This remediation not only resolves the immediate stability issue but also prevents potential exploitation scenarios where an attacker could potentially leverage the preemption violation to gain unauthorized access or cause system-wide disruptions during power management operations, particularly in virtualized environments where Xen hypervisor integration is critical for system functionality and security.