userfaultfd: mcopy_atomic: return -ENOENT when no compatible VMA found

The memory mapping of a process may change between #PF event and the
call to mcopy_atomic that comes to resolve the page fault.  In such
case, there will be no VMA covering the range passed to mcopy_atomic or
the VMA will not have userfaultfd context.

To allow uffd monitor to distinguish those case from other errors, let's
return -ENOENT instead of -EINVAL.

Note, that despite availability of UFFD_EVENT_UNMAP there still might be
race between the processing of UFFD_EVENT_UNMAP and outstanding
mcopy_atomic in case of non-cooperative uffd usage.

[rppt@linux.vnet.ibm.com: update cases returning -ENOENT]
  Link: http://lkml.kernel.org/r/20170207150249.GA6709@rapoport-lnx
[aarcange@redhat.com: merge fix]
[akpm@linux-foundation.org: fix the merge fix]
Link: http://lkml.kernel.org/r/1485542673-24387-5-git-send-email-rppt@linux.vnet.ibm.com
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 28 insertions, 30 deletions

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 1e5c2f94e8a3..9f0ad2a4f102 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -197,22 +197,25 @@ retry:
          * retry, dst_vma will be set to NULL and we must lookup again.
          */
         if (!dst_vma) {
-                err = -EINVAL;
+                err = -ENOENT;
                 dst_vma = find_vma(dst_mm, dst_start);
                 if (!dst_vma || !is_vm_hugetlb_page(dst_vma))
                         goto out_unlock;
-
-                if (vma_hpagesize != vma_kernel_pagesize(dst_vma))
-                        goto out_unlock;
-
                 /*
-                 * Make sure the remaining dst range is both valid and
-                 * fully within a single existing vma.
+                 * Only allow __mcopy_atomic_hugetlb on userfaultfd
+                 * registered ranges.
                  */
+                if (!dst_vma->vm_userfaultfd_ctx.ctx)
+                        goto out_unlock;
+
                 if (dst_start < dst_vma->vm_start ||
                     dst_start + len > dst_vma->vm_end)
                         goto out_unlock;
 
+                err = -EINVAL;
+                if (vma_hpagesize != vma_kernel_pagesize(dst_vma))
+                        goto out_unlock;
+
                 vm_shared = dst_vma->vm_flags & VM_SHARED;
         }
 
@@ -221,12 +224,6 @@ retry:
                 goto out_unlock;
 
         /*
-         * Only allow __mcopy_atomic_hugetlb on userfaultfd registered ranges.
-         */
-        if (!dst_vma->vm_userfaultfd_ctx.ctx)
-                goto out_unlock;
-
-        /*
          * If not shared, ensure the dst_vma has a anon_vma.
          */
         err = -ENOMEM;
@@ -404,22 +401,35 @@ retry:
          * Make sure the vma is not shared, that the dst range is
          * both valid and fully within a single existing vma.
          */
-        err = -EINVAL;
+        err = -ENOENT;
         dst_vma = find_vma(dst_mm, dst_start);
         if (!dst_vma)
                 goto out_unlock;
         /*
-         * shmem_zero_setup is invoked in mmap for MAP_ANONYMOUS|MAP_SHARED but
-         * it will overwrite vm_ops, so vma_is_anonymous must return false.
+         * Be strict and only allow __mcopy_atomic on userfaultfd
+         * registered ranges to prevent userland errors going
+         * unnoticed. As far as the VM consistency is concerned, it
+         * would be perfectly safe to remove this check, but there's
+         * no useful usage for __mcopy_atomic ouside of userfaultfd
+         * registered ranges. This is after all why these are ioctls
+         * belonging to the userfaultfd and not syscalls.
          */
-        if (WARN_ON_ONCE(vma_is_anonymous(dst_vma) &&
-            dst_vma->vm_flags & VM_SHARED))
+        if (!dst_vma->vm_userfaultfd_ctx.ctx)
                 goto out_unlock;
 
         if (dst_start < dst_vma->vm_start ||
             dst_start + len > dst_vma->vm_end)
                 goto out_unlock;
 
+        err = -EINVAL;
+        /*
+         * shmem_zero_setup is invoked in mmap for MAP_ANONYMOUS|MAP_SHARED but
+         * it will overwrite vm_ops, so vma_is_anonymous must return false.
+         */
+        if (WARN_ON_ONCE(vma_is_anonymous(dst_vma) &&
+            dst_vma->vm_flags & VM_SHARED))
+                goto out_unlock;
+
         /*
          * If this is a HUGETLB vma, pass off to appropriate routine
          */
@@ -427,18 +437,6 @@ retry:
                 return  __mcopy_atomic_hugetlb(dst_mm, dst_vma, dst_start,
                                                 src_start, len, zeropage);
 
-        /*
-         * Be strict and only allow __mcopy_atomic on userfaultfd
-         * registered ranges to prevent userland errors going
-         * unnoticed. As far as the VM consistency is concerned, it
-         * would be perfectly safe to remove this check, but there's
-         * no useful usage for __mcopy_atomic ouside of userfaultfd
-         * registered ranges. This is after all why these are ioctls
-         * belonging to the userfaultfd and not syscalls.
-         */
-        if (!dst_vma->vm_userfaultfd_ctx.ctx)
-                goto out_unlock;
-
         if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
                 goto out_unlock;