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    Subject[PATCH 3.16 06/26] mm: migrate: Close race between migration completion and mprotect
    Date
    3.16-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Mel Gorman <mgorman@suse.de>

    commit d3cb8bf6081b8b7a2dabb1264fe968fd870fa595 upstream.

    A migration entry is marked as write if pte_write was true at the time the
    entry was created. The VMA protections are not double checked when migration
    entries are being removed as mprotect marks write-migration-entries as
    read. It means that potentially we take a spurious fault to mark PTEs write
    again but it's straight-forward. However, there is a race between write
    migrations being marked read and migrations finishing. This potentially
    allows a PTE to be write that should have been read. Close this race by
    double checking the VMA permissions using maybe_mkwrite when migration
    completes.

    [torvalds@linux-foundation.org: use maybe_mkwrite]
    Signed-off-by: Mel Gorman <mgorman@suse.de>
    Acked-by: Rik van Riel <riel@redhat.com>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    mm/migrate.c | 5 ++++-
    1 file changed, 4 insertions(+), 1 deletion(-)

    --- a/mm/migrate.c
    +++ b/mm/migrate.c
    @@ -146,8 +146,11 @@ static int remove_migration_pte(struct p
    pte = pte_mkold(mk_pte(new, vma->vm_page_prot));
    if (pte_swp_soft_dirty(*ptep))
    pte = pte_mksoft_dirty(pte);
    +
    + /* Recheck VMA as permissions can change since migration started */
    if (is_write_migration_entry(entry))
    - pte = pte_mkwrite(pte);
    + pte = maybe_mkwrite(pte, vma);
    +
    #ifdef CONFIG_HUGETLB_PAGE
    if (PageHuge(new)) {
    pte = pte_mkhuge(pte);



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