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In the Linux kernel’s x86 memory management, a race condition in the `should_flush_tlb()` function could cause necessary TLB (Translation Lookaside Buffer) flushes to be incorrectly skipped. The issue occurs during a context switch when a new memory space (mm) is loaded. After the new page table base register (CR3) is set via load_new_mm_cr3(), the CPU requires TLB flushes for the new mm. However, the `should_flush_tlb()` logic, which determines if an Inter-Processor Interrupt (IPI) for a TLB flush should be sent, incorrectly suppresses the flush in the brief window before the `cpu_tlbstate.loaded_mm` pointer is updated to the new mm. During this window, the CPU’s state is LOADED_MM_SWITCHING. The vulnerability stems from overly aggressive optimizations to lazily manage the mm_cpumask. The fix modifies `should_flush_tlb()` to recognize the `LOADED_MM_SWITCHING` state, ensuring an IPI is always sent when needed during this critical period. A memory barrier was also added to guarantee proper ordering of writes and observations of the `loaded_mm` and `is_lazy` states, closing the race condition window.
Platform: Linux Kernel
Version: Affected versions
Vulnerability: TLB Flush Bypass
Severity: Not Specified
date: 05/20/2025
Prediction: Patch Available
What Undercode Say:
Showing bash commands and codes related to the blog
grep -r "should_flush_tlb" /usr/src/linux-kernel-version/ cat /proc/cpuinfo | grep "bugs" dmesg | grep "TLB"
// Code snippet from the fix in switch_mm_irqs_off() cpumask_set_cpu(cpu, mm_cpumask(next)); next_tlb_gen = atomic64_read(&next->context.tlb_gen); choose_new_asid(next, next_tlb_gen, &new_asid, &need_flush); load_new_mm_cr3(need_flush); this_cpu_write(cpu_tlbstate.loaded_mm, next); // Critical update
How Exploit:
An exploit would require precise timing to leverage the race condition window where a CPU has switched to a new page table (CR3) but the kernel’s tracking state (loaded_mm) hasn’t been updated. A malicious actor on a multi-core system could attempt to execute code on a target core during this window. If successful, the core might incorrectly use stale TLB entries from a previous process’s memory space, potentially accessing or modifying memory it shouldn’t have access to, leading to information disclosure or privilege escalation.
Protection from this CVE
Apply the official kernel patch from kernel.org that fixes the `should_flush_tlb()` logic. Update your Linux kernel to a version that includes the commit resolving this issue. For distributions, install the latest stable kernel package provided by your vendor. Regularly update systems and reboot to ensure the patched kernel is active.
Impact
The primary impact is potential corruption of memory isolation between processes. This could lead to information disclosure, where sensitive data from one process is leaked to another, or a privilege escalation if kernel memory mappings are affected. The flaw requires local access and exploits a narrow timing window on multi-processor systems, making reliable exploitation challenging but possible.
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Sources:
Reported By: nvd.nist.gov
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