CVE-2025-37964

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 Software

References

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