CVE-2023-52476: Linux Kernel Vulnerability - Resolving 'perf/x86/lbr: Filter vsyscall addresses'

A recent vulnerability (CVE-2023-52476) has been identified in the Linux kernel, which affects the performance subsystem of the x86 architecture (perf/x86/lbr). The vulnerability revolves around the incorrect handling of vsyscall addresses while Last Branch Record (LBR) sampling is active, which led to a kernel panic and a crash.

In this post, we will discuss the details of this vulnerability, the code snippet responsible for the issue, the fix applied to alleviate the situation, and the referenced resources. Let's dive in!

Linux Kernel Vulnerability

The problem arises when a vsyscall is made while LBR sampling is active. If the vsyscall is interrupted (NMI) for perf sampling, the following call sequence can occur (most recent at top):

__insn_get_emulate_prefix()
insn_get_emulate_prefix()
insn_get_prefixes()
insn_get_opcode()
decode_branch_type()
get_branch_type()
intel_pmu_lbr_filter()
intel_pmu_handle_irq()
perf_event_nmi_handler()

Inside the __insn_get_emulate_prefix() function at frame , a macro is called

peek_nbyte_next(insn_byte_t, insn, i)

During the execution of this macro, this dereference occurs

(insn)->next_byte

On inspecting registers at this point, the value of the next_byte field is found to be the address of the vsyscall made, such as the location of the vsyscall version of gettimeofday() at xffffffffff600000. The access to an address in the vsyscall region will trigger an "oops" due to an unhandled page fault.

The Fix

To address this vulnerability, a patch was applied that filters for vsyscalls during the determination of branch types. The fix returns a "none" branch if a kernel address is in the vsyscall region. This eliminates the crash caused by the mishandling of vsyscall addresses during LBR sampling.

The original data source and references to this vulnerability can be found below

1. CVE-2023-52476 on NVD
2. Linux Kernel Git Commit: Filter vsyscall addresses

Conclusion

The CVE-2023-52476 vulnerability highlights the importance of thoroughly testing and validating kernel-level code, particularly during interrupts and low-level operations. While this specific vulnerability has been resolved, it serves as a reminder for security researchers and Linux kernel developers alike that subtle bugs that lead to severe consequences can still slip through the cracks. By following best coding practices, continuously reviewing code changes, and collaborating effectively, the Linux community can further strengthen the security and reliability of the Linux kernel.

Timeline

Published on: 02/29/2024 06:15:45 UTC
Last modified on: 12/19/2024 08:20:32 UTC