x86/mm: Eliminate window where TLB flushes may be inadvertently skipped

Overview

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

• Linux >= 848b5815177582de0e1d0118725378e0fbadca20 < 12f703811af043d32b1c8a30001b2fa04d5cd0ac
• Linux >= b47002ed65ade940839b7f439ff4a194e7d5ec28 < 02ad4ce144bd27f71f583f667fdf3b3ba0753477
• Linux >= a04fe3bfc71e28009e20357b79df1e8ef7c9d600 < d41072906abec8bb8e01ed16afefbaa558908c89
• Linux >= 3dbe889a1b829b4c07e0836ff853fe649e51ce4f < d87392094f96e162fa5fa5a8640d70cc0952806f
• Linux >= 6db2526c1d694c91c6e05e2f186c085e9460f202 < 399ec9ca8fc4999e676ff89a90184ec40031cf59
• Linux >= 6db2526c1d694c91c6e05e2f186c085e9460f202 < fea4e317f9e7e1f449ce90dedc27a2d2a95bee5a
• Linux d1347977661342cb09a304a17701eb2d4aa21dec
• Linux >= 5.15.179 < 5.15.183
• Linux >= 6.1.129 < 6.1.139
• Linux >= 6.6.79 < 6.6.91
• Linux >= 6.12.16 < 6.12.29
• Linux >= 6.13.4 < 6.14
• Linux 6.14

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.