{"dataType":"CVE_RECORD","dataVersion":"5.1","cveMetadata":{"cveId":"CVE-2024-46787","assignerOrgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","state":"PUBLISHED","assignerShortName":"Linux","dateReserved":"2024-09-11T15:12:18.278Z","datePublished":"2024-09-18T07:12:43.264Z","dateUpdated":"2025-05-04T09:34:18.496Z"},"containers":{"cna":{"providerMetadata":{"orgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","shortName":"Linux","dateUpdated":"2025-05-04T09:34:18.496Z"},"descriptions":[{"lang":"en","value":"In the Linux kernel, the following vulnerability has been resolved:\n\nuserfaultfd: fix checks for huge PMDs\n\nPatch series \"userfaultfd: fix races around pmd_trans_huge() check\", v2.\n\nThe pmd_trans_huge() code in mfill_atomic() is wrong in three different\nways depending on kernel version:\n\n1. The pmd_trans_huge() check is racy and can lead to a BUG_ON() (if you hit\n   the right two race windows) - I've tested this in a kernel build with\n   some extra mdelay() calls. See the commit message for a description\n   of the race scenario.\n   On older kernels (before 6.5), I think the same bug can even\n   theoretically lead to accessing transhuge page contents as a page table\n   if you hit the right 5 narrow race windows (I haven't tested this case).\n2. As pointed out by Qi Zheng, pmd_trans_huge() is not sufficient for\n   detecting PMDs that don't point to page tables.\n   On older kernels (before 6.5), you'd just have to win a single fairly\n   wide race to hit this.\n   I've tested this on 6.1 stable by racing migration (with a mdelay()\n   patched into try_to_migrate()) against UFFDIO_ZEROPAGE - on my x86\n   VM, that causes a kernel oops in ptlock_ptr().\n3. On newer kernels (>=6.5), for shmem mappings, khugepaged is allowed\n   to yank page tables out from under us (though I haven't tested that),\n   so I think the BUG_ON() checks in mfill_atomic() are just wrong.\n\nI decided to write two separate fixes for these (one fix for bugs 1+2, one\nfix for bug 3), so that the first fix can be backported to kernels\naffected by bugs 1+2.\n\n\nThis patch (of 2):\n\nThis fixes two issues.\n\nI discovered that the following race can occur:\n\n  mfill_atomic                other thread\n  ============                ============\n                              <zap PMD>\n  pmdp_get_lockless() [reads none pmd]\n  <bail if trans_huge>\n  <if none:>\n                              <pagefault creates transhuge zeropage>\n    __pte_alloc [no-op]\n                              <zap PMD>\n  <bail if pmd_trans_huge(*dst_pmd)>\n  BUG_ON(pmd_none(*dst_pmd))\n\nI have experimentally verified this in a kernel with extra mdelay() calls;\nthe BUG_ON(pmd_none(*dst_pmd)) triggers.\n\nOn kernels newer than commit 0d940a9b270b (\"mm/pgtable: allow\npte_offset_map[_lock]() to fail\"), this can't lead to anything worse than\na BUG_ON(), since the page table access helpers are actually designed to\ndeal with page tables concurrently disappearing; but on older kernels\n(<=6.4), I think we could probably theoretically race past the two\nBUG_ON() checks and end up treating a hugepage as a page table.\n\nThe second issue is that, as Qi Zheng pointed out, there are other types\nof huge PMDs that pmd_trans_huge() can't catch: devmap PMDs and swap PMDs\n(in particular, migration PMDs).\n\nOn <=6.4, this is worse than the first issue: If mfill_atomic() runs on a\nPMD that contains a migration entry (which just requires winning a single,\nfairly wide race), it will pass the PMD to pte_offset_map_lock(), which\nassumes that the PMD points to a page table.\n\nBreakage follows: First, the kernel tries to take the PTE lock (which will\ncrash or maybe worse if there is no \"struct page\" for the address bits in\nthe migration entry PMD - I think at least on X86 there usually is no\ncorresponding \"struct page\" thanks to the PTE inversion mitigation, amd64\nlooks different).\n\nIf that didn't crash, the kernel would next try to write a PTE into what\nit wrongly thinks is a page table.\n\nAs part of fixing these issues, get rid of the check for pmd_trans_huge()\nbefore __pte_alloc() - that's redundant, we're going to have to check for\nthat after the __pte_alloc() anyway.\n\nBackport note: pmdp_get_lockless() is pmd_read_atomic() in older kernels."}],"affected":[{"product":"Linux","vendor":"Linux","defaultStatus":"unaffected","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["mm/userfaultfd.c"],"versions":[{"version":"c1a4de99fada21e2e9251e52cbb51eff5aadc757","lessThan":"3c6b4bcf37845c9359aed926324bed66bdd2448d","status":"affected","versionType":"git"},{"version":"c1a4de99fada21e2e9251e52cbb51eff5aadc757","lessThan":"98cc18b1b71e23fe81a5194ed432b20c2d81a01a","status":"affected","versionType":"git"},{"version":"c1a4de99fada21e2e9251e52cbb51eff5aadc757","lessThan":"71c186efc1b2cf1aeabfeff3b9bd5ac4c5ac14d8","status":"affected","versionType":"git"}]},{"product":"Linux","vendor":"Linux","defaultStatus":"affected","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["mm/userfaultfd.c"],"versions":[{"version":"4.3","status":"affected"},{"version":"0","lessThan":"4.3","status":"unaffected","versionType":"semver"},{"version":"6.6.51","lessThanOrEqual":"6.6.*","status":"unaffected","versionType":"semver"},{"version":"6.10.10","lessThanOrEqual":"6.10.*","status":"unaffected","versionType":"semver"},{"version":"6.11","lessThanOrEqual":"*","status":"unaffected","versionType":"original_commit_for_fix"}]}],"cpeApplicability":[{"nodes":[{"operator":"OR","negate":false,"cpeMatch":[{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.3","versionEndExcluding":"6.6.51"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.3","versionEndExcluding":"6.10.10"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.3","versionEndExcluding":"6.11"}]}]}],"references":[{"url":"https://git.kernel.org/stable/c/3c6b4bcf37845c9359aed926324bed66bdd2448d"},{"url":"https://git.kernel.org/stable/c/98cc18b1b71e23fe81a5194ed432b20c2d81a01a"},{"url":"https://git.kernel.org/stable/c/71c186efc1b2cf1aeabfeff3b9bd5ac4c5ac14d8"}],"title":"userfaultfd: fix checks for huge PMDs","x_generator":{"engine":"bippy-1.2.0"}},"adp":[{"metrics":[{"other":{"type":"ssvc","content":{"timestamp":"2024-09-29T14:28:53.563201Z","id":"CVE-2024-46787","options":[{"Exploitation":"none"},{"Automatable":"no"},{"Technical Impact":"partial"}],"role":"CISA Coordinator","version":"2.0.3"}}}],"title":"CISA ADP Vulnrichment","providerMetadata":{"orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","shortName":"CISA-ADP","dateUpdated":"2024-09-29T14:29:08.018Z"}}]}}