{"id":"CVE-2026-31397","state":"PUBLISHED","title":"mm/huge_memory: fix use of NULL folio in move_pages_huge_pmd()","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nmm/huge_memory: fix use of NULL folio in move_pages_huge_pmd()\n\nmove_pages_huge_pmd() handles UFFDIO_MOVE for both normal THPs and huge\nzero pages.  For the huge zero page path, src_folio is explicitly set to\nNULL, and is used as a sentinel to skip folio operations like lock and\nrmap.\n\nIn the huge zero page branch, src_folio is NULL, so folio_mk_pmd(NULL,\npgprot) passes NULL through folio_pfn() and page_to_pfn().  With\nSPARSEMEM_VMEMMAP this silently produces a bogus PFN, installing a PMD\npointing to non-existent physical memory.  On other memory models it is a\nNULL dereference.\n\nUse page_folio(src_page) to obtain the valid huge zero folio from the\npage, which was obtained from pmd_page() and remains valid throughout.\n\nAfter commit d82d09e48219 (\"mm/huge_memory: mark PMD mappings of the huge\nzero folio special\"), moved huge zero PMDs must remain special so\nvm_normal_page_pmd() continues to treat them as special mappings.\n\nmove_pages_huge_pmd() currently reconstructs the destination PMD in the\nhuge zero page branch, which drops PMD state such as pmd_special() on\narchitectures with CONFIG_ARCH_HAS_PTE_SPECIAL.  As a result,\nvm_normal_page_pmd() can treat the moved huge zero PMD as a normal page\nand corrupt its refcount.\n\nInstead of reconstructing the PMD from the folio, derive the destination\nentry from src_pmdval after pmdp_huge_clear_flush(), then handle the PMD\nmetadata the same way move_huge_pmd() does for moved entries by marking it\nsoft-dirty and clearing uffd-wp.","publishedAt":"2026-04-03T15:16:01.427Z","updatedAt":"2026-08-05T12:22:06.974Z","severity":{"score":7.8,"label":"High","version":"3.1","vector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H","source":"Linux"},"assigner":"Linux","products":["Linux Linux"],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"e3981db444a0a18d350d9f92e3f2e8d489b54211","status":"affected","lessThan":"f3caaee0f9e489fd2282d4ce45791dc8aed2da62","versionType":"git"},{"version":"e3981db444a0a18d350d9f92e3f2e8d489b54211","status":"affected","lessThan":"e3133d0986dc5a231d5419167dbac65312b28b41","versionType":"git"},{"version":"e3981db444a0a18d350d9f92e3f2e8d489b54211","status":"affected","lessThan":"fae654083bfa409bb2244f390232e2be47f05bfc","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["mm/huge_memory.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"6.16","status":"affected"},{"version":"0","status":"unaffected","lessThan":"6.16","versionType":"semver"},{"version":"6.18.20","status":"unaffected","lessThanOrEqual":"6.18.*","versionType":"semver"},{"version":"6.19.10","status":"unaffected","lessThanOrEqual":"6.19.*","versionType":"semver"},{"version":"7.0","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["mm/huge_memory.c"]}],"references":["https://git.kernel.org/stable/c/f3caaee0f9e489fd2282d4ce45791dc8aed2da62","https://git.kernel.org/stable/c/e3133d0986dc5a231d5419167dbac65312b28b41","https://git.kernel.org/stable/c/fae654083bfa409bb2244f390232e2be47f05bfc"],"weaknesses":[],"ingestedAt":"2026-10-29T20:11:40.391Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/31xxx/CVE-2026-31397.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-31397","state":"PUBLISHED","datePublished":"2026-04-03T15:16:01.427Z","dateUpdated":"2026-08-05T12:22:06.974Z","assignerShortName":"Linux","assignerOrgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateReserved":"2026-03-09T15:48:24.085Z"},"containers":{"cna":{"title":"mm/huge_memory: fix use of NULL folio in move_pages_huge_pmd()","descriptions":[{"lang":"en","value":"In the Linux kernel, the following vulnerability has been resolved:\n\nmm/huge_memory: fix use of NULL folio in move_pages_huge_pmd()\n\nmove_pages_huge_pmd() handles UFFDIO_MOVE for both normal THPs and huge\nzero pages.  For the huge zero page path, src_folio is explicitly set to\nNULL, and is used as a sentinel to skip folio operations like lock and\nrmap.\n\nIn the huge zero page branch, src_folio is NULL, so folio_mk_pmd(NULL,\npgprot) passes NULL through folio_pfn() and page_to_pfn().  With\nSPARSEMEM_VMEMMAP this silently produces a bogus PFN, installing a PMD\npointing to non-existent physical memory.  On other memory models it is a\nNULL dereference.\n\nUse page_folio(src_page) to obtain the valid huge zero folio from the\npage, which was obtained from pmd_page() and remains valid throughout.\n\nAfter commit d82d09e48219 (\"mm/huge_memory: mark PMD mappings of the huge\nzero folio special\"), moved huge zero PMDs must remain special so\nvm_normal_page_pmd() continues to treat them as special mappings.\n\nmove_pages_huge_pmd() currently reconstructs the destination PMD in the\nhuge zero page branch, which drops PMD state such as pmd_special() on\narchitectures with CONFIG_ARCH_HAS_PTE_SPECIAL.  As a result,\nvm_normal_page_pmd() can treat the moved huge zero PMD as a normal page\nand corrupt its refcount.\n\nInstead of reconstructing the PMD from the folio, derive the destination\nentry from src_pmdval after pmdp_huge_clear_flush(), then handle the PMD\nmetadata the same way move_huge_pmd() does for moved entries by marking it\nsoft-dirty and clearing uffd-wp."}],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"e3981db444a0a18d350d9f92e3f2e8d489b54211","status":"affected","lessThan":"f3caaee0f9e489fd2282d4ce45791dc8aed2da62","versionType":"git"},{"version":"e3981db444a0a18d350d9f92e3f2e8d489b54211","status":"affected","lessThan":"e3133d0986dc5a231d5419167dbac65312b28b41","versionType":"git"},{"version":"e3981db444a0a18d350d9f92e3f2e8d489b54211","status":"affected","lessThan":"fae654083bfa409bb2244f390232e2be47f05bfc","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["mm/huge_memory.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"6.16","status":"affected"},{"version":"0","status":"unaffected","lessThan":"6.16","versionType":"semver"},{"version":"6.18.20","status":"unaffected","lessThanOrEqual":"6.18.*","versionType":"semver"},{"version":"6.19.10","status":"unaffected","lessThanOrEqual":"6.19.*","versionType":"semver"},{"version":"7.0","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["mm/huge_memory.c"]}],"references":[{"url":"https://git.kernel.org/stable/c/f3caaee0f9e489fd2282d4ce45791dc8aed2da62"},{"url":"https://git.kernel.org/stable/c/e3133d0986dc5a231d5419167dbac65312b28b41"},{"url":"https://git.kernel.org/stable/c/fae654083bfa409bb2244f390232e2be47f05bfc"}],"metrics":[{"cvssV3_1":{"version":"3.1","vectorString":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H","baseScore":7.8,"baseSeverity":"HIGH"},"scenarios":[{"lang":"en","value":"AV:L - The vulnerable path is reached through local `userfaultfd` ioctls, specifically `UFFDIO_MOVE`, on the attacker's own address space.\nAC:L - An attacker can create aligned anonymous VMAs, materialize a huge zero PMD with reads, register the destination with userfaultfd, and issue `UFFDIO_MOVE`; no uncontrollable race is required.\nPR:L - A basic unprivileged local user can create a userspace-only userfaultfd with `UFFD_USER_MODE_ONLY` and exercise `UFFDIO_MOVE` on their own memory.\nUI:N - Exploitation is driven entirely by the attacker's own syscalls and memory accesses and does not require a victim user action.\nS:U - The impact is within the local kernel and the attacking process's memory-management context, with no separate security authority crossed.\nC:H - The bug can install a malformed huge PMD and the fix notes refcount corruption when huge zero PMDs lose special state; treating this as kernel memory corruption makes high confidentiality impact defensible.\nI:H - The malformed PMD/refcount corruption affects kernel-managed memory structures and can plausibly be developed beyond a crash into memory modification or control-flow compromise.\nA:H - On some memory models this is a direct NULL dereference, and on common sparse-vmemmap systems it installs a bogus PMD that can crash the kernel during access or later VM teardown."}]}],"providerMetadata":{"shortName":"Linux","orgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateUpdated":"2026-08-05T12:22:06.974Z"},"cpeApplicability":[{"nodes":[{"operator":"OR","negate":false,"cpeMatch":[{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"6.16","versionEndExcluding":"6.18.20"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"6.16","versionEndExcluding":"6.19.10"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"6.16","versionEndExcluding":"7.0"}]}]}],"x_generator":{"engine":"bippy-1.2.0"}}}}}