x86/mm: Eliminate window where TLB flushes may be inadvertently skipped
- Published
- Last modified
- Where the record stands: published, rejected by its publisher, or superseded by another id.What do these mean? →
- published
- Whether CISA lists it in its Known Exploited Vulnerabilities catalog: proof that attackers are using it.What do these mean? →
- Not listed in CISA KEV
- Fix
- Fixed in 5.15.183, 6.1.139, 6.6.91, 6.12.29, 6.14, and 1 more
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
Summary
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.
Timeline
- CVE published
- Record last updated
Affected versions
- Linux 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 Linux
6.14
- KernelLinux
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