• Andrea Arcangeli's avatar
    userfaultfd: shmem: allocate anonymous memory for MAP_PRIVATE shmem · 5b51072e
    Andrea Arcangeli authored
    Userfaultfd did not create private memory when UFFDIO_COPY was invoked
    on a MAP_PRIVATE shmem mapping.  Instead it wrote to the shmem file,
    even when that had not been opened for writing.  Though, fortunately,
    that could only happen where there was a hole in the file.
    
    Fix the shmem-backed implementation of UFFDIO_COPY to create private
    memory for MAP_PRIVATE mappings.  The hugetlbfs-backed implementation
    was already correct.
    
    This change is visible to userland, if userfaultfd has been used in
    unintended ways: so it introduces a small risk of incompatibility, but
    is necessary in order to respect file permissions.
    
    An app that uses UFFDIO_COPY for anything like postcopy live migration
    won't notice the difference, and in fact it'll run faster because there
    will be no copy-on-write and memory waste in the tmpfs pagecache
    anymore.
    
    Userfaults on MAP_PRIVATE shmem keep triggering only on file holes like
    before.
    
    The real zeropage can also be built on a MAP_PRIVATE shmem mapping
    through UFFDIO_ZEROPAGE and that's safe because the zeropage pte is
    never dirty, in turn even an mprotect upgrading the vma permission from
    PROT_READ to PROT_READ|PROT_WRITE won't make the zeropage pte writable.
    
    Link: http://lkml.kernel.org/r/20181126173452.26955-3-aarcange@redhat.com
    Fixes: 4c27fe4c
    
     ("userfaultfd: shmem: add shmem_mcopy_atomic_pte for userfaultfd support")
    Signed-off-by: default avatarAndrea Arcangeli <aarcange@redhat.com>
    Reported-by: default avatarMike Rapoport <rppt@linux.ibm.com>
    Reviewed-by: default avatarHugh Dickins <hughd@google.com>
    Cc: <stable@vger.kernel.org>
    Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
    Cc: Jann Horn <jannh@google.com>
    Cc: Mike Kravetz <mike.kravetz@oracle.com>
    Cc: Peter Xu <peterx@redhat.com>
    Cc: stable@vger.kernel.org
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    5b51072e
userfaultfd.c 15.5 KB