Description

In the Linux kernel, the following vulnerability has been resolved: x86/mm: Eliminate window where TLB flushes may be inadvertently skipped tl;dr: There is a window in the mm switching code where the new CR3 is set and the CPU should be getting TLB flushes for the new mm. But should_flush_tlb() has a bug and suppresses the flush. Fix it by widening the window where should_flush_tlb() sends an IPI. Long Version: === History === There were a few things leading up to this. First, updating mm_cpumask() was observed to be too expensive, so it was made lazier. But being lazy caused too many unnecessary IPIs to CPUs due to the now-lazy mm_cpumask(). So code was added to cull mm_cpumask() periodically[2]. But that culling was a bit too aggressive and skipped sending TLB flushes to CPUs that need them. So here we are again. === Problem === The too-aggressive code in should_flush_tlb() strikes in this window: // Turn on IPIs for this CPU/mm combination, but only // if should_flush_tlb() agrees: 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); // ^ After 'need_flush' is set to false, IPIs *MUST* // be sent to this CPU and not be ignored. this_cpu_write(cpu_tlbstate.loaded_mm, next); // ^ Not until this point does should_flush_tlb() // become true! should_flush_tlb() will suppress TLB flushes between load_new_mm_cr3() and writing to 'loaded_mm', which is a window where they should not be suppressed. Whoops. === Solution === Thankfully, the fuzzy "just about to write CR3" window is already marked with loaded_mm==LOADED_MM_SWITCHING. Simply checking for that state in should_flush_tlb() is sufficient to ensure that the CPU is targeted with an IPI. This will cause more TLB flush IPIs. But the window is relatively small and I do not expect this to cause any kind of measurable performance impact. Update the comment where LOADED_MM_SWITCHING is written since it grew yet another user. Peter Z also raised a concern that should_flush_tlb() might not observe 'loaded_mm' and 'is_lazy' in the same order that switch_mm_irqs_off() writes them. Add a barrier to ensure that they are observed in the order they are written.

Affected Products 11

References 7

• git.kernel.org https://git.kernel.org/stable/c/02ad4ce144bd27f71f583f667fdf3b3ba0753477
  Patch
• git.kernel.org https://git.kernel.org/stable/c/12f703811af043d32b1c8a30001b2fa04d5cd0ac
  Patch
• git.kernel.org https://git.kernel.org/stable/c/399ec9ca8fc4999e676ff89a90184ec40031cf59
  Patch
• git.kernel.org https://git.kernel.org/stable/c/d41072906abec8bb8e01ed16afefbaa558908c89
  Patch
• git.kernel.org https://git.kernel.org/stable/c/d87392094f96e162fa5fa5a8640d70cc0952806f
  Patch
• git.kernel.org https://git.kernel.org/stable/c/fea4e317f9e7e1f449ce90dedc27a2d2a95bee5a
  Patch
• lists.debian.org https://lists.debian.org/debian-lts-announce/2025/08/msg00010.html
  Third Party Advisory

Remediation

• git.kernel.org https://git.kernel.org/stable/c/02ad4ce144bd27f71f583f667fdf3b3ba0753477
  Patch
• git.kernel.org https://git.kernel.org/stable/c/12f703811af043d32b1c8a30001b2fa04d5cd0ac
  Patch
• git.kernel.org https://git.kernel.org/stable/c/399ec9ca8fc4999e676ff89a90184ec40031cf59
  Patch
• git.kernel.org https://git.kernel.org/stable/c/d41072906abec8bb8e01ed16afefbaa558908c89
  Patch
• git.kernel.org https://git.kernel.org/stable/c/d87392094f96e162fa5fa5a8640d70cc0952806f
  Patch
• git.kernel.org https://git.kernel.org/stable/c/fea4e317f9e7e1f449ce90dedc27a2d2a95bee5a
  Patch