{ "dataType": "CVE_RECORD", "dataVersion": "5.2", "cveMetadata": { "cveId": "CVE-2025-21984", "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "state": "PUBLISHED", "assignerShortName": "Linux", "dateReserved": "2024-12-29T08:45:45.799Z", "datePublished": "2025-04-01T15:47:11.523Z", "dateUpdated": "2026-08-05T11:55:50.769Z" }, "containers": { "cna": { "providerMetadata": { "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "shortName": "Linux", "dateUpdated": "2026-08-05T11:55:50.769Z" }, "descriptions": [ { "lang": "en", "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nmm: fix kernel BUG when userfaultfd_move encounters swapcache\n\nuserfaultfd_move() checks whether the PTE entry is present or a\nswap entry.\n\n- If the PTE entry is present, move_present_pte() handles folio\n migration by setting:\n\n src_folio->index = linear_page_index(dst_vma, dst_addr);\n\n- If the PTE entry is a swap entry, move_swap_pte() simply copies\n the PTE to the new dst_addr.\n\nThis approach is incorrect because, even if the PTE is a swap entry,\nit can still reference a folio that remains in the swap cache.\n\nThis creates a race window between steps 2 and 4.\n 1. add_to_swap: The folio is added to the swapcache.\n 2. try_to_unmap: PTEs are converted to swap entries.\n 3. pageout: The folio is written back.\n 4. Swapcache is cleared.\nIf userfaultfd_move() occurs in the window between steps 2 and 4,\nafter the swap PTE has been moved to the destination, accessing the\ndestination triggers do_swap_page(), which may locate the folio in\nthe swapcache. However, since the folio's index has not been updated\nto match the destination VMA, do_swap_page() will detect a mismatch.\n\nThis can result in two critical issues depending on the system\nconfiguration.\n\nIf KSM is disabled, both small and large folios can trigger a BUG\nduring the add_rmap operation due to:\n\n page_pgoff(folio, page) != linear_page_index(vma, address)\n\n[ 13.336953] page: refcount:6 mapcount:1 mapping:00000000f43db19c index:0xffffaf150 pfn:0x4667c\n[ 13.337520] head: order:2 mapcount:1 entire_mapcount:0 nr_pages_mapped:1 pincount:0\n[ 13.337716] memcg:ffff00000405f000\n[ 13.337849] anon flags: 0x3fffc0000020459(locked|uptodate|dirty|owner_priv_1|head|swapbacked|node=0|zone=0|lastcpupid=0xffff)\n[ 13.338630] raw: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361\n[ 13.338831] raw: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000\n[ 13.339031] head: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361\n[ 13.339204] head: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000\n[ 13.339375] head: 03fffc0000000202 fffffdffc0199f01 ffffffff00000000 0000000000000001\n[ 13.339546] head: 0000000000000004 0000000000000000 00000000ffffffff 0000000000000000\n[ 13.339736] page dumped because: VM_BUG_ON_PAGE(page_pgoff(folio, page) != linear_page_index(vma, address))\n[ 13.340190] ------------[ cut here ]------------\n[ 13.340316] kernel BUG at mm/rmap.c:1380!\n[ 13.340683] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP\n[ 13.340969] Modules linked in:\n[ 13.341257] CPU: 1 UID: 0 PID: 107 Comm: a.out Not tainted 6.14.0-rc3-gcf42737e247a-dirty #299\n[ 13.341470] Hardware name: linux,dummy-virt (DT)\n[ 13.341671] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)\n[ 13.341815] pc : __page_check_anon_rmap+0xa0/0xb0\n[ 13.341920] lr : __page_check_anon_rmap+0xa0/0xb0\n[ 13.342018] sp : ffff80008752bb20\n[ 13.342093] x29: ffff80008752bb20 x28: fffffdffc0199f00 x27: 0000000000000001\n[ 13.342404] x26: 0000000000000000 x25: 0000000000000001 x24: 0000000000000001\n[ 13.342575] x23: 0000ffffaf0d0000 x22: 0000ffffaf0d0000 x21: fffffdffc0199f00\n[ 13.342731] x20: fffffdffc0199f00 x19: ffff000006210700 x18: 00000000ffffffff\n[ 13.342881] x17: 6c203d2120296567 x16: 6170202c6f696c6f x15: 662866666f67705f\n[ 13.343033] x14: 6567617028454741 x13: 2929737365726464 x12: ffff800083728ab0\n[ 13.343183] x11: ffff800082996bf8 x10: 0000000000000fd7 x9 : ffff80008011bc40\n[ 13.343351] x8 : 0000000000017fe8 x7 : 00000000fffff000 x6 : ffff8000829eebf8\n[ 13.343498] x5 : c0000000fffff000 x4 : 0000000000000000 x3 : 0000000000000000\n[ 13.343645] x2 : 0000000000000000 x1 : ffff0000062db980 x0 : 000000000000005f\n[ 13.343876] Call trace:\n[ 13.344045] __page_check_anon_rmap+0xa0/0xb0 (P)\n[ 13.344234] folio_add_anon_rmap_ptes+0x22c/0x320\n[ 13.344333] do_swap_page+0x1060/0x1400\n[ 13.344417] __handl\n---truncated---" } ], "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 vulnerability is reached only through local syscalls — `userfaultfd(2)`/`/dev/userfaultfd` plus the `UFFDIO_MOVE` ioctl and `madvise`/page-fault activity on the attacker's own address space. There is no network- or adjacent-reachable path.\nAC:L - The attacker creates and drives both sides of the race himself: `madvise(MADV_PAGEOUT)` deterministically pushes a chosen folio into the swapcache with swap PTEs, while a second thread loops `UFFDIO_MOVE` over the same address, retrying until the window is hit. A trivial reproducer accompanied the fix, and swap (zram on Android/embedded, swapfiles on servers) is a standard configuration.\nPR:L - Only an unprivileged local user is needed — `userfaultfd_syscall_allowed()` always permits `UFFD_USER_MODE_ONLY` contexts regardless of the `vm.unprivileged_userfaultfd` sysctl, and `UFFDIO_MOVE` acts on the caller's own mm with no capability check. Android app sandboxes routinely have userfaultfd access for ART GC compaction.\nUI:N - The attacking process performs every step itself — register uffd, page out, move the PTE, then touch the destination. No victim action or interaction is required.\nS:U - The corruption stays within the kernel's own memory-management state and the attacking process's mm; there is no hypervisor, IOMMU, or other security-authority boundary crossed. This is standard local kernel compromise, so scope is unchanged.\nC:H - The folio ends up mapped in `dst_vma` while `folio->mapping` still references the source anon_vma and `folio->index` is stale, breaking the invariant that a mapped folio's anon_vma cannot be freed; a subsequently freed/reused anon_vma dereferenced by rmap walks yields a use-after-free read primitive over attacker-groomed slab contents, and mis-addressed rmap walks can expose unrelated mapping state.\nI:H - The same dangling anon_vma plus stale index corrupts reverse-mapping metadata for a live, writable anonymous folio, which is the classic shape for heap-spray-driven write primitives and control-flow hijack; with KSM enabled the bug additionally causes a silent unexpected CoW that breaks UFFDIO_MOVE atomicity and corrupts the userspace GC heap that depends on it.\nA:H - The reported outcome is `kernel BUG at mm/rmap.c` in `__page_check_anon_rmap()` reached from `do_swap_page()` — an oops taken with page-table locks held and IRQs disabled (`exited with preempt_count 2`), which panics the box on `panic_on_oops` and wedges the mm otherwise. Even without `CONFIG_DEBUG_VM`, the resulting rmap corruption and dangling anon_vma produce later crashes." } ] } ], "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": "adef440691bab824e39c1b17382322d195e1fab0", "lessThan": "4e9507246298fd6f1ca7bb42ef01a6e34fb93684", "status": "affected", "versionType": "git" }, { "version": "adef440691bab824e39c1b17382322d195e1fab0", "lessThan": "b1e11bd86c0943bb7624efebdc384340a50ad683", "status": "affected", "versionType": "git" }, { "version": "adef440691bab824e39c1b17382322d195e1fab0", "lessThan": "c50f8e6053b0503375c2975bf47f182445aebb4c", "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": "6.8", "status": "affected" }, { "version": "0", "lessThan": "6.8", "status": "unaffected", "versionType": "semver" }, { "version": "6.12.20", "lessThanOrEqual": "6.12.*", "status": "unaffected", "versionType": "semver" }, { "version": "6.13.8", "lessThanOrEqual": "6.13.*", "status": "unaffected", "versionType": "semver" }, { "version": "6.14", "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": "6.8", "versionEndExcluding": "6.12.20" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.8", "versionEndExcluding": "6.13.8" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.8", "versionEndExcluding": "6.14" } ] } ] } ], "references": [ { "url": "https://git.kernel.org/stable/c/4e9507246298fd6f1ca7bb42ef01a6e34fb93684" }, { "url": "https://git.kernel.org/stable/c/b1e11bd86c0943bb7624efebdc384340a50ad683" }, { "url": "https://git.kernel.org/stable/c/c50f8e6053b0503375c2975bf47f182445aebb4c" } ], "title": "mm: fix kernel BUG when userfaultfd_move encounters swapcache", "x_generator": { "engine": "bippy-1.2.0" } }, "adp": [ { "problemTypes": [ { "descriptions": [ { "type": "CWE", "cweId": "CWE-362", "lang": "en", "description": "CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')" } ] } ], "metrics": [ { "cvssV3_1": { "scope": "UNCHANGED", "version": "3.1", "baseScore": 4.7, "attackVector": "LOCAL", "baseSeverity": "MEDIUM", "vectorString": "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H", "integrityImpact": "NONE", "userInteraction": "NONE", "attackComplexity": "HIGH", "availabilityImpact": "HIGH", "privilegesRequired": "LOW", "confidentialityImpact": "NONE" } }, { "other": { "type": "ssvc", "content": { "timestamp": "2025-10-01T17:14:21.959942Z", "id": "CVE-2025-21984", "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": "2025-10-01T17:14:24.524Z" } } ] } }