mm/huge_memory: fix dereferencing invalid pmd migration entry

JIRA: https://issues.redhat.com/browse/RHEL-96372
CVE: CVE-2025-37958
Conflicts:
  * context conflicts due to RHEL-9 missing upstream commit 29e847d
    ("mm/rmap: integrate PMD-mapped folio splitting into pagewalk loop")
    which is part of a bigger series "Reclaim lazyfree THP without splitting"
    and is irrelevant to the fixlet for this CVE.

commit be6e843
Author: Gavin Guo <[email protected]>
Date:   Mon Apr 21 19:35:36 2025 +0800

    mm/huge_memory: fix dereferencing invalid pmd migration entry

    When migrating a THP, concurrent access to the PMD migration entry during
    a deferred split scan can lead to an invalid address access, as
    illustrated below.  To prevent this invalid access, it is necessary to
    check the PMD migration entry and return early.  In this context, there is
    no need to use pmd_to_swp_entry and pfn_swap_entry_to_page to verify the
    equality of the target folio.  Since the PMD migration entry is locked, it
    cannot be served as the target.

    Mailing list discussion and explanation from Hugh Dickins: "An anon_vma
    lookup points to a location which may contain the folio of interest, but
    might instead contain another folio: and weeding out those other folios is
    precisely what the "folio != pmd_folio((*pmd)" check (and the "risk of
    replacing the wrong folio" comment a few lines above it) is for."

    BUG: unable to handle page fault for address: ffffea60001db008
    CPU: 0 UID: 0 PID: 2199114 Comm: tee Not tainted 6.14.0+ #4 NONE
    Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
    RIP: 0010:split_huge_pmd_locked+0x3b5/0x2b60
    Call Trace:
    <TASK>
    try_to_migrate_one+0x28c/0x3730
    rmap_walk_anon+0x4f6/0x770
    unmap_folio+0x196/0x1f0
    split_huge_page_to_list_to_order+0x9f6/0x1560
    deferred_split_scan+0xac5/0x12a0
    shrinker_debugfs_scan_write+0x376/0x470
    full_proxy_write+0x15c/0x220
    vfs_write+0x2fc/0xcb0
    ksys_write+0x146/0x250
    do_syscall_64+0x6a/0x120
    entry_SYSCALL_64_after_hwframe+0x76/0x7e

    The bug is found by syzkaller on an internal kernel, then confirmed on
    upstream.

    Link: https://lkml.kernel.org/r/[email protected]
    Link: https://lore.kernel.org/all/[email protected]/
    Link: https://lore.kernel.org/all/[email protected]/
    Fixes: 84c3fc4 ("mm: thp: check pmd migration entry in common path")
    Signed-off-by: Gavin Guo <[email protected]>
    Acked-by: David Hildenbrand <[email protected]>
    Acked-by: Hugh Dickins <[email protected]>
    Acked-by: Zi Yan <[email protected]>
    Reviewed-by: Gavin Shan <[email protected]>
    Cc: Florent Revest <[email protected]>
    Cc: Matthew Wilcox (Oracle) <[email protected]>
    Cc: Miaohe Lin <[email protected]>
    Cc: <[email protected]>
    Signed-off-by: Andrew Morton <[email protected]>

Signed-off-by: Rafael Aquini <[email protected]>

Author: Rafael Aquini

mm/huge_memory.c

@@ -2312,6 +2312,7 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 {
 	spinlock_t *ptl;
 	struct mmu_notifier_range range;
+	bool pmd_migration;
 
 	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma->vm_mm,
 				address & HPAGE_PMD_MASK,
@@ -2326,13 +2327,13 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 	VM_BUG_ON(freeze && !folio);
 	VM_WARN_ON_ONCE(folio && !folio_test_locked(folio));
 
-	if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) ||
-	    is_pmd_migration_entry(*pmd)) {
+	pmd_migration = is_pmd_migration_entry(*pmd);
+	if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) || pmd_migration) {
 		/*
-		 * It's safe to call pmd_page when folio is set because it's
-		 * guaranteed that pmd is present.
+		 * Do not apply pmd_folio() to a migration entry; and folio lock
+		 * guarantees that it must be of the wrong folio anyway.
 		 */
-		if (folio && folio != page_folio(pmd_page(*pmd)))
+		if (folio && (pmd_migration || folio != page_folio(pmd_page(*pmd))))
 			goto out;
 		__split_huge_pmd_locked(vma, pmd, range.start, freeze);
 	}