From: Andrea Arcangeli aarcange@redhat.com Subject: userfaultfd: shmem: allocate anonymous memory for MAP_PRIVATE shmem

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: 4c27fe4c4c84 ("userfaultfd: shmem: add shmem_mcopy_atomic_pte for userfaultfd support") Signed-off-by: Andrea Arcangeli aarcange@redhat.com Reported-by: Mike Rapoport rppt@linux.ibm.com Reviewed-by: Hugh 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: Andrew Morton akpm@linux-foundation.org ---

mm/userfaultfd.c | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-)

--- a/mm/userfaultfd.c~userfaultfd-shmem-allocate-anonymous-memory-for-map_private-shmem +++ a/mm/userfaultfd.c @@ -380,7 +380,17 @@ static __always_inline ssize_t mfill_ato { ssize_t err;

- if (vma_is_anonymous(dst_vma)) { + /* + * The normal page fault path for a shmem will invoke the + * fault, fill the hole in the file and COW it right away. The + * result generates plain anonymous memory. So when we are + * asked to fill an hole in a MAP_PRIVATE shmem mapping, we'll + * generate anonymous memory directly without actually filling + * the hole. For the MAP_PRIVATE case the robustness check + * only happens in the pagetable (to verify it's still none) + * and not in the radix tree. + */ + if (!(dst_vma->vm_flags & VM_SHARED)) { if (!zeropage) err = mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma, dst_addr, src_addr, page); @@ -489,7 +499,8 @@ retry: * dst_vma. */ err = -ENOMEM; - if (vma_is_anonymous(dst_vma) && unlikely(anon_vma_prepare(dst_vma))) + if (!(dst_vma->vm_flags & VM_SHARED) && + unlikely(anon_vma_prepare(dst_vma))) goto out_unlock;

while (src_addr < src_start + len) { _