CVE-2025-21984: In the Linux kernel, the following vulnerability has been resolved:  mm: fix kernel BUG when userfaultfd_move encounters swapcache  userfaultfd_move() checks whether the PTE entry is present or a swap entry.  - If the PTE entry is present, move_present_pte() handles folio   migration by setting:    src_folio->index = linear_page_index(dst_vma, dst_addr);  - If the PTE entry is a swap entry, move_swap_pte() simply copies   the PTE to the new dst_addr.  This approach is incorrect because, even if the PTE is a swap entry, it can still reference a folio that remains in the swap cache.  This creates a race window between steps 2 and 4.  1. add_to_swap: The folio is added to the swapcache.  2. try_to_unmap: PTEs are converted to swap entries.  3. pageout: The folio is written back.  4. Swapcache is cleared. If userfaultfd_move() occurs in the window between steps 2 and 4, after the swap PTE has been moved to the destination, accessing the destination triggers do_swap_page(), which may locate the folio in the swapcache. However, since the folio's index has not been updated to match the destination VMA, do_swap_page() will detect a mismatch.  This can result in two critical issues depending on the system configuration.  If KSM is disabled, both small and large folios can trigger a BUG during the add_rmap operation due to:   page_pgoff(folio, page) != linear_page_index(vma, address)  [   13.336953] page: refcount:6 mapcount:1 mapping:00000000f43db19c index:0xffffaf150 pfn:0x4667c [   13.337520] head: order:2 mapcount:1 entire_mapcount:0 nr_pages_mapped:1 pincount:0 [   13.337716] memcg:ffff00000405f000 [   13.337849] anon flags: 0x3fffc0000020459(locked|uptodate|dirty|owner_priv_1|head|swapbacked|node=0|zone=0|lastcpupid=0xffff) [   13.338630] raw: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361 [   13.338831] raw: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000 [   13.339031] head: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361 [   13.339204] head: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000 [   13.339375] head: 03fffc0000000202 fffffdffc0199f01 ffffffff00000000 0000000000000001 [   13.339546] head: 0000000000000004 0000000000000000 00000000ffffffff 0000000000000000 [   13.339736] page dumped because: VM_BUG_ON_PAGE(page_pgoff(folio, page) != linear_page_index(vma, address)) [   13.340190] ------------[ cut here ]------------ [   13.340316] kernel BUG at mm/rmap.c:1380! [   13.340683] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP [   13.340969] Modules linked in: [   13.341257] CPU: 1 UID: 0 PID: 107 Comm: a.out Not tainted 6.14.0-rc3-gcf42737e247a-dirty #299 [   13.341470] Hardware name: linux,dummy-virt (DT) [   13.341671] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--) [   13.341815] pc : __page_check_anon_rmap+0xa0/0xb0 [   13.341920] lr : __page_check_anon_rmap+0xa0/0xb0 [   13.342018] sp : ffff80008752bb20 [   13.342093] x29: ffff80008752bb20 x28: fffffdffc0199f00 x27: 0000000000000001 [   13.342404] x26: 0000000000000000 x25: 0000000000000001 x24: 0000000000000001 [   13.342575] x23: 0000ffffaf0d0000 x22: 0000ffffaf0d0000 x21: fffffdffc0199f00 [   13.342731] x20: fffffdffc0199f00 x19: ffff000006210700 x18: 00000000ffffffff [   13.342881] x17: 6c203d2120296567 x16: 6170202c6f696c6f x15: 662866666f67705f [   13.343033] x14: 6567617028454741 x13: 2929737365726464 x12: ffff800083728ab0 [   13.343183] x11: ffff800082996bf8 x10: 0000000000000fd7 x9 : ffff80008011bc40 [   13.343351] x8 : 0000000000017fe8 x7 : 00000000fffff000 x6 : ffff8000829eebf8 [   13.343498] x5 : c0000000fffff000 x4 : 0000000000000000 x3 : 0000000000000000 [   13.343645] x2 : 0000000000000000 x1 : ffff0000062db980 x0 : 000000000000005f [   13.343876] Call trace: [   13.344045]  __page_check_anon_rmap+0xa0/0xb0 (P) [   13.344234]  folio_add_anon_rmap_ptes+0x22c/0x320 [   13.344333]  do_swap_page+0x1060/0x1400 [   13.344417]  __handl ---truncated---