{"id":"CVE-2025-37964","summary":"x86/mm: Eliminate window where TLB flushes may be inadvertently skipped","details":"In the Linux kernel, the following vulnerability has been resolved:\n\nx86/mm: Eliminate window where TLB flushes may be inadvertently skipped\n\ntl;dr: There is a window in the mm switching code where the new CR3 is\nset and the CPU should be getting TLB flushes for the new mm.  But\nshould_flush_tlb() has a bug and suppresses the flush.  Fix it by\nwidening the window where should_flush_tlb() sends an IPI.\n\nLong Version:\n\n=== History ===\n\nThere were a few things leading up to this.\n\nFirst, updating mm_cpumask() was observed to be too expensive, so it was\nmade lazier.  But being lazy caused too many unnecessary IPIs to CPUs\ndue to the now-lazy mm_cpumask().  So code was added to cull\nmm_cpumask() periodically[2].  But that culling was a bit too aggressive\nand skipped sending TLB flushes to CPUs that need them.  So here we are\nagain.\n\n=== Problem ===\n\nThe too-aggressive code in should_flush_tlb() strikes in this window:\n\n\t// Turn on IPIs for this CPU/mm combination, but only\n\t// if should_flush_tlb() agrees:\n\tcpumask_set_cpu(cpu, mm_cpumask(next));\n\n\tnext_tlb_gen = atomic64_read(&next-\u003econtext.tlb_gen);\n\tchoose_new_asid(next, next_tlb_gen, &new_asid, &need_flush);\n\tload_new_mm_cr3(need_flush);\n\t// ^ After 'need_flush' is set to false, IPIs *MUST*\n\t// be sent to this CPU and not be ignored.\n\n        this_cpu_write(cpu_tlbstate.loaded_mm, next);\n\t// ^ Not until this point does should_flush_tlb()\n\t// become true!\n\nshould_flush_tlb() will suppress TLB flushes between load_new_mm_cr3()\nand writing to 'loaded_mm', which is a window where they should not be\nsuppressed.  Whoops.\n\n=== Solution ===\n\nThankfully, the fuzzy \"just about to write CR3\" window is already marked\nwith loaded_mm==LOADED_MM_SWITCHING.  Simply checking for that state in\nshould_flush_tlb() is sufficient to ensure that the CPU is targeted with\nan IPI.\n\nThis will cause more TLB flush IPIs.  But the window is relatively small\nand I do not expect this to cause any kind of measurable performance\nimpact.\n\nUpdate the comment where LOADED_MM_SWITCHING is written since it grew\nyet another user.\n\nPeter Z also raised a concern that should_flush_tlb() might not observe\n'loaded_mm' and 'is_lazy' in the same order that switch_mm_irqs_off()\nwrites them.  Add a barrier to ensure that they are observed in the\norder they are written.","modified":"2026-04-16T04:41:48.487673481Z","published":"2025-05-20T16:01:56.013Z","database_specific":{"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/37xxx/CVE-2025-37964.json","cna_assigner":"Linux"},"references":[{"type":"WEB","url":"https://git.kernel.org/stable/c/02ad4ce144bd27f71f583f667fdf3b3ba0753477"},{"type":"WEB","url":"https://git.kernel.org/stable/c/12f703811af043d32b1c8a30001b2fa04d5cd0ac"},{"type":"WEB","url":"https://git.kernel.org/stable/c/399ec9ca8fc4999e676ff89a90184ec40031cf59"},{"type":"WEB","url":"https://git.kernel.org/stable/c/d41072906abec8bb8e01ed16afefbaa558908c89"},{"type":"WEB","url":"https://git.kernel.org/stable/c/d87392094f96e162fa5fa5a8640d70cc0952806f"},{"type":"WEB","url":"https://git.kernel.org/stable/c/fea4e317f9e7e1f449ce90dedc27a2d2a95bee5a"},{"type":"WEB","url":"https://lists.debian.org/debian-lts-announce/2025/08/msg00010.html"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/37xxx/CVE-2025-37964.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2025-37964"},{"type":"PACKAGE","url":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"848b5815177582de0e1d0118725378e0fbadca20"},{"fixed":"12f703811af043d32b1c8a30001b2fa04d5cd0ac"}]},{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"b47002ed65ade940839b7f439ff4a194e7d5ec28"},{"fixed":"02ad4ce144bd27f71f583f667fdf3b3ba0753477"}]},{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"a04fe3bfc71e28009e20357b79df1e8ef7c9d600"},{"fixed":"d41072906abec8bb8e01ed16afefbaa558908c89"}]},{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"3dbe889a1b829b4c07e0836ff853fe649e51ce4f"},{"fixed":"d87392094f96e162fa5fa5a8640d70cc0952806f"}]},{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"6db2526c1d694c91c6e05e2f186c085e9460f202"},{"fixed":"399ec9ca8fc4999e676ff89a90184ec40031cf59"},{"fixed":"fea4e317f9e7e1f449ce90dedc27a2d2a95bee5a"}]},{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"0"},{"last_affected":"d1347977661342cb09a304a17701eb2d4aa21dec"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2025-37964.json"}}],"schema_version":"1.7.5"}