diff options
authorAndrea Arcangeli <aarcange@redhat.com>2017-09-08 16:12:42 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2017-09-08 18:26:47 -0700
commit656710a60e3693911bee3a355d2f2bbae3faba33 (patch)
parent98c70baad4067cf3bfb90c26b3b01cb3eab621e2 (diff)
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.
+ */
goto out;
+ }
* Check that we can return VM_FAULT_RETRY.