The patch titled Subject: mm/khugepaged: fix ->anon_vma race has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-khugepaged-fix-anon_vma-race.patch
This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...
This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------ From: Jann Horn jannh@google.com Subject: mm/khugepaged: fix ->anon_vma race Date: Wed, 11 Jan 2023 14:33:51 +0100
If an ->anon_vma is attached to the VMA, collapse_and_free_pmd() requires it to be locked. retract_page_tables() bails out if an ->anon_vma is attached, but does this check before holding the mmap lock (as the comment above the check explains).
If we racily merge an existing ->anon_vma (shared with a child process) from a neighboring VMA, subsequent rmap traversals on pages belonging to the child will be able to see the page tables that we are concurrently removing while assuming that nothing else can access them.
Repeat the ->anon_vma check once we hold the mmap lock to ensure that there really is no concurrent page table access.
Link: https://lkml.kernel.org/r/20230111133351.807024-1-jannh@google.com Fixes: f3f0e1d2150b ("khugepaged: add support of collapse for tmpfs/shmem pages") Signed-off-by: Jann Horn jannh@google.com Reported-by: Zach O'Keefe zokeefe@google.com Cc: David Hildenbrand david@redhat.com Cc: Kirill A. Shutemov kirill.shutemov@linux.intel.com Cc: Yang Shi shy828301@gmail.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
mm/khugepaged.c | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-)
--- a/mm/khugepaged.c~mm-khugepaged-fix-anon_vma-race +++ a/mm/khugepaged.c @@ -1642,7 +1642,7 @@ static int retract_page_tables(struct ad * has higher cost too. It would also probably require locking * the anon_vma. */ - if (vma->anon_vma) { + if (READ_ONCE(vma->anon_vma)) { result = SCAN_PAGE_ANON; goto next; } @@ -1671,6 +1671,18 @@ static int retract_page_tables(struct ad if ((cc->is_khugepaged || is_target) && mmap_write_trylock(mm)) { /* + * Re-check whether we have an ->anon_vma, because + * collapse_and_free_pmd() requires that either no + * ->anon_vma exists or the anon_vma is locked. + * We already checked ->anon_vma above, but that check + * is racy because ->anon_vma can be populated under the + * mmap lock in read mode. + */ + if (vma->anon_vma) { + result = SCAN_PAGE_ANON; + goto unlock_next; + } + /* * When a vma is registered with uffd-wp, we can't * recycle the pmd pgtable because there can be pte * markers installed. Skip it only, so the rest mm/vma _
Patches currently in -mm which might be from jannh@google.com are
mm-khugepaged-fix-anon_vma-race.patch