Revert "userfaultfd: don't fail on unrecognized features"

This is a proposal to revert commit 914eedcb.

I found this when writing a simple UFFDIO_API test to be the first unit
test in this set.  Two things breaks with the commit:

  - UFFDIO_API check was lost and missing.  According to man page, the
  kernel should reject ioctl(UFFDIO_API) if uffdio_api.api != 0xaa.  This
  check is needed if the api version will be extended in the future, or
  user app won't be able to identify which is a new kernel.

  - Feature flags checks were removed, which means UFFDIO_API with a
  feature that does not exist will also succeed.  According to the man
  page, we should (and it makes sense) to reject ioctl(UFFDIO_API) if
  unknown features passed in.

Link: https://lore.kernel.org/r/20220722201513.1624158-1-axelrasmussen@google.com
Link: https://lkml.kernel.org/r/20230412163922.327282-2-peterx@redhat.com
Fixes: 914eedcb ("userfaultfd: don't fail on unrecognized features")
Signed-off-by: Peter Xu <peterx@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Dmitry Safonov <0x7f454c46@gmail.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Zach O'Keefe <zokeefe@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/userfaultfd.c

@@ -1955,8 +1955,10 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
 	ret = -EFAULT;
 	if (copy_from_user(&uffdio_api, buf, sizeof(uffdio_api)))
 		goto out;
-	/* Ignore unsupported features (userspace built against newer kernel) */
-	features = uffdio_api.features & UFFD_API_FEATURES;
+	features = uffdio_api.features;
+	ret = -EINVAL;
+	if (uffdio_api.api != UFFD_API || (features & ~UFFD_API_FEATURES))
+		goto err_out;
 	ret = -EPERM;
 	if ((features & UFFD_FEATURE_EVENT_FORK) && !capable(CAP_SYS_PTRACE))
 		goto err_out;