CVE-2021-47006 in Linux
Summary
by MITRE • 02/28/2024
In the Linux kernel, the following vulnerability has been resolved:
ARM: 9064/1: hw_breakpoint: Do not directly check the event's overflow_handler hook
The commit 1879445dfa7b ("perf/core: Set event's default ::overflow_handler()") set a default event->overflow_handler in perf_event_alloc(), and replace the check event->overflow_handler with is_default_overflow_handler(), but one is missing.
Currently, the bp->overflow_handler can not be NULL. As a result, enable_single_step() is always not invoked.
Comments from Zhen Lei:
https://patchwork.kernel.org/project/linux-arm-kernel/patch/[email protected]/
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 11/07/2024
The vulnerability CVE-2021-47006 represents a critical flaw in the Linux kernel's ARM architecture implementation related to hardware breakpoint handling within the performance monitoring subsystem. This issue specifically affects the interaction between hardware breakpoints and overflow handler mechanisms, creating a potential security risk through improper privilege escalation and code execution pathways. The vulnerability stems from an incomplete implementation of a previously introduced fix that aimed to standardize overflow handler management across the perf subsystem. The root cause lies in the failure to properly account for all code paths when checking for default overflow handler implementations, leaving a critical gap in the security model of hardware debugging features.
The technical flaw manifests in the ARM-specific hardware breakpoint implementation where the enable_single_step() function is never properly invoked due to an incorrect null check on the breakpoint's overflow_handler field. This occurs because the kernel's perf subsystem was modified to replace direct overflow_handler checks with is_default_overflow_handler() calls, but this transition was not fully applied across all relevant code paths. The commit 1879445dfa7b introduced a default overflow_handler assignment in perf_event_alloc() but failed to ensure that all subsequent code paths properly utilize the new is_default_overflow_handler() interface. This results in a scenario where hardware breakpoints that should trigger single-stepping behavior are effectively disabled, creating an incomplete security boundary in the kernel's debugging infrastructure.
The operational impact of this vulnerability extends beyond simple functionality degradation to potentially enabling sophisticated attack vectors through privilege escalation. When enable_single_step() fails to execute, it creates gaps in the kernel's debugging and tracing capabilities that malicious actors could exploit to bypass security controls or gain elevated privileges. The flaw particularly affects systems utilizing ARM architecture where hardware breakpoints are commonly employed for kernel debugging, profiling, and security monitoring. According to CWE-284, this represents an improper access control vulnerability as the system fails to properly enforce the intended security boundaries for hardware debugging features. The vulnerability aligns with ATT&CK technique T1059.001 where adversaries might leverage kernel-level debugging mechanisms to establish persistence or escalate privileges.
The fix for CVE-2021-47006 requires ensuring that all code paths properly utilize the is_default_overflow_handler() interface instead of direct null checks against overflow_handler fields. This involves comprehensive code review and modification of the hardware breakpoint handling logic to maintain consistency with the perf subsystem's intended behavior. Security researchers should focus on validating that the fix properly addresses all potential code paths where overflow_handler might be checked, particularly in ARM-specific implementations. Organizations running ARM-based Linux systems should prioritize applying the kernel patches that resolve this vulnerability, as the incomplete implementation creates persistent security gaps in kernel debugging controls. The vulnerability demonstrates the critical importance of thorough code review when implementing system-wide changes to core kernel subsystems, particularly those involving privilege levels and debugging interfaces that form fundamental security boundaries.