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(...

Classification

CVE ID: CVE-2025-37964

Affected Products

Vendor: Linux

Product: Linux

Exploit Prediction Scoring System (EPSS)

EPSS Score: 0.06% (probability of being exploited)

EPSS Percentile: 17.55% (scored less or equal to compared to others)

EPSS Date: 2025-06-18 (when was this score calculated)

References

https://nvd.nist.gov/vuln/detail/CVE-2025-37964
https://git.kernel.org/stable/c/12f703811af043d32b1c8a30001b2fa04d5cd0ac
https://git.kernel.org/stable/c/02ad4ce144bd27f71f583f667fdf3b3ba0753477
https://git.kernel.org/stable/c/d41072906abec8bb8e01ed16afefbaa558908c89
https://git.kernel.org/stable/c/d87392094f96e162fa5fa5a8640d70cc0952806f
https://git.kernel.org/stable/c/399ec9ca8fc4999e676ff89a90184ec40031cf59
https://git.kernel.org/stable/c/fea4e317f9e7e1f449ce90dedc27a2d2a95bee5a

Timeline