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.
Acked-by: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
---
fs/userfaultfd.c | 20 +++++++++++++++++++-
1 file changed, 19 insertions(+), 1 deletion(-)
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 272c21d8d532..186831c80a75 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -394,8 +394,26 @@ 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.
--
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