userfaultfd: non-cooperative: closing the uffd without triggering SIGBUS

This is an enhancement to avoid a non cooperative userfaultfd manager
having to unregister all regions before it can close the uffd after all
userfaultfd activity completed.

The UFFDIO_UNREGISTER would serialize against the handle_userfault by
taking the mmap_sem for writing, but we can simply repeat the page fault
if we detect the uffd was closed and so the regular page fault paths
should takeover.

Link: http://lkml.kernel.org/r/20170823181227.19926-1-aarcange@redhat.com
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Mike Rapoport <rppt@linux.vnet.ibm.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, 19 insertions, 1 deletions

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 5419e7da82ba..ef4b48d1ea42 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -381,8 +381,26 @@ int handle_userfault(struct vm_fault *vmf, unsigned long reason)
          * in __get_user_pages if userfaultfd_release waits on the
          * caller of handle_userfault to release the mmap_sem.
          */
-        if (unlikely(ACCESS_ONCE(ctx->released)))
+        if (unlikely(ACCESS_ONCE(ctx->released))) {
+                /*
+                 * Don't return VM_FAULT_SIGBUS in this case, so a non
+                 * cooperative manager can close the uffd after the
+                 * last UFFDIO_COPY, without risking to trigger an
+                 * involuntary SIGBUS if the process was starting the
+                 * userfaultfd while the userfaultfd was still armed
+                 * (but after the last UFFDIO_COPY). If the uffd
+                 * wasn't already closed when the userfault reached
+                 * this point, that would normally be solved by
+                 * userfaultfd_must_wait returning 'false'.
+                 *
+                 * If we were to return VM_FAULT_SIGBUS here, the non
+                 * cooperative manager would be instead forced to
+                 * always call UFFDIO_UNREGISTER before it can safely
+                 * close the uffd.
+                 */
+                ret = VM_FAULT_NOPAGE;
                 goto out;
+        }
 
         /*
          * Check that we can return VM_FAULT_RETRY.