CVE-2025-39844: Software Security Vulnerability Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-fv7f-vxxp-c378",
  "modified" : "2026-05-12T15:31:09Z",
  "published" : "2025-09-22T21:30:17Z",
  "aliases" : [ "CVE-2025-39844" ],
  "details" : "In the Linux kernel, the following vulnerability has been resolved:\n\nmm: move page table sync declarations to linux/pgtable.h\n\nDuring our internal testing, we started observing intermittent boot\nfailures when the machine uses 4-level paging and has a large amount of\npersistent memory:\n\n  BUG: unable to handle page fault for address: ffffe70000000034\n  #PF: supervisor write access in kernel mode\n  #PF: error_code(0x0002) - not-present page\n  PGD 0 P4D 0 \n  Oops: 0002 [#1] SMP NOPTI\n  RIP: 0010:__init_single_page+0x9/0x6d\n  Call Trace:\n   <TASK>\n   __init_zone_device_page+0x17/0x5d\n   memmap_init_zone_device+0x154/0x1bb\n   pagemap_range+0x2e0/0x40f\n   memremap_pages+0x10b/0x2f0\n   devm_memremap_pages+0x1e/0x60\n   dev_dax_probe+0xce/0x2ec [device_dax]\n   dax_bus_probe+0x6d/0xc9\n   [... snip ...]\n   </TASK>\n\nIt turns out that the kernel panics while initializing vmemmap (struct\npage array) when the vmemmap region spans two PGD entries, because the new\nPGD entry is only installed in init_mm.pgd, but not in the page tables of\nother tasks.\n\nAnd looking at __populate_section_memmap():\n  if (vmemmap_can_optimize(altmap, pgmap))                                \n          // does not sync top level page tables\n          r = vmemmap_populate_compound_pages(pfn, start, end, nid, pgmap);\n  else                                                                    \n          // sync top level page tables in x86\n          r = vmemmap_populate(start, end, nid, altmap);\n\nIn the normal path, vmemmap_populate() in arch/x86/mm/init_64.c\nsynchronizes the top level page table (See commit 9b861528a801 (\"x86-64,\nmem: Update all PGDs for direct mapping and vmemmap mapping changes\")) so\nthat all tasks in the system can see the new vmemmap area.\n\nHowever, when vmemmap_can_optimize() returns true, the optimized path\nskips synchronization of top-level page tables.  This is because\nvmemmap_populate_compound_pages() is implemented in core MM code, which\ndoes not handle synchronization of the top-level page tables.  Instead,\nthe core MM has historically relied on each architecture to perform this\nsynchronization manually.\n\nWe're not the first party to encounter a crash caused by not-sync'd top\nlevel page tables: earlier this year, Gwan-gyeong Mun attempted to address\nthe issue [1] [2] after hitting a kernel panic when x86 code accessed the\nvmemmap area before the corresponding top-level entries were synced.  At\nthat time, the issue was believed to be triggered only when struct page\nwas enlarged for debugging purposes, and the patch did not get further\nupdates.\n\nIt turns out that current approach of relying on each arch to handle the\npage table sync manually is fragile because 1) it's easy to forget to sync\nthe top level page table, and 2) it's also easy to overlook that the\nkernel should not access the vmemmap and direct mapping areas before the\nsync.\n\n# The solution: Make page table sync more code robust and harder to miss\n\nTo address this, Dave Hansen suggested [3] [4] introducing\n{pgd,p4d}_populate_kernel() for updating kernel portion of the page tables\nand allow each architecture to explicitly perform synchronization when\ninstalling top-level entries.  With this approach, we no longer need to\nworry about missing the sync step, reducing the risk of future\nregressions.\n\nThe new interface reuses existing ARCH_PAGE_TABLE_SYNC_MASK,\nPGTBL_P*D_MODIFIED and arch_sync_kernel_mappings() facility used by\nvmalloc and ioremap to synchronize page tables.\n\npgd_populate_kernel() looks like this:\nstatic inline void pgd_populate_kernel(unsigned long addr, pgd_t *pgd,\n                                       p4d_t *p4d)\n{\n        pgd_populate(&init_mm, pgd, p4d);\n        if (ARCH_PAGE_TABLE_SYNC_MASK & PGTBL_PGD_MODIFIED)\n                arch_sync_kernel_mappings(addr, addr);\n}\n\nIt is worth noting that vmalloc() and apply_to_range() carefully\nsynchronizes page tables by calling p*d_alloc_track() and\narch_sync_kernel_mappings(), and thus they are not affected by\n---truncated---",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2025-39844"
  }, {
    "type" : "WEB",
    "url" : "https://cert-portal.siemens.com/productcert/html/ssa-032379.html"
  }, {
    "type" : "WEB",
    "url" : "https://git.kernel.org/stable/c/469f9d22751472b81eaaf8a27fcdb5a70741c342"
  }, {
    "type" : "WEB",
    "url" : "https://git.kernel.org/stable/c/4f7537772011fad832f83d6848f8eab282545bef"
  }, {
    "type" : "WEB",
    "url" : "https://git.kernel.org/stable/c/6797a8b3f71b2cb558b8771a03450dc3e004e453"
  }, {
    "type" : "WEB",
    "url" : "https://git.kernel.org/stable/c/732e62212f49d549c91071b4da7942ee3058f7a2"
  }, {
    "type" : "WEB",
    "url" : "https://git.kernel.org/stable/c/7cc183f2e67d19b03ee5c13a6664b8c6cc37ff9d"
  }, {
    "type" : "WEB",
    "url" : "https://git.kernel.org/stable/c/eceb44e1f94bd641b2a4e8c09b64c797c4eabc15"
  }, {
    "type" : "WEB",
    "url" : "https://lists.debian.org/debian-lts-announce/2025/10/msg00008.html"
  } ],
  "database_specific" : {
    "cwe_ids" : [ ],
    "severity" : "MODERATE",
    "github_reviewed" : false,
    "github_reviewed_at" : null,
    "nvd_published_at" : "2025-09-19T16:15:43Z"
  }
}