[PATCH v3 26/28] userfaultfd: wp: declare _UFFDIO_WRITEPROTECT conditionally

[Peter Xu]

Only declare _UFFDIO_WRITEPROTECT if the user specified
UFFDIO_REGISTER_MODE_WP and if all the checks passed.  Then when the
user registers regions with shmem/hugetlbfs we won't expose the new
ioctl to them.  Even with complete anonymous memory range, we'll only
expose the new WP ioctl bit if the register mode has MODE_WP.

Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
 fs/userfaultfd.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index f1f61a0278c2..7f87e9e4fb9b 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1456,14 +1456,24 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
 	up_write(&mm->mmap_sem);
 	mmput(mm);
 	if (!ret) {
+		__u64 ioctls_out;
+
+		ioctls_out = basic_ioctls ? UFFD_API_RANGE_IOCTLS_BASIC :
+		    UFFD_API_RANGE_IOCTLS;
+
+		/*
+		 * Declare the WP ioctl only if the WP mode is
+		 * specified and all checks passed with the range
+		 */
+		if (!(uffdio_register.mode & UFFDIO_REGISTER_MODE_WP))
+			ioctls_out &= ~((__u64)1 << _UFFDIO_WRITEPROTECT);
+
 		/*
 		 * Now that we scanned all vmas we can already tell
 		 * userland which ioctls methods are guaranteed to
 		 * succeed on this range.
 		 */
-		if (put_user(basic_ioctls ? UFFD_API_RANGE_IOCTLS_BASIC :
-			     UFFD_API_RANGE_IOCTLS,
-			     &user_uffdio_register->ioctls))
+		if (put_user(ioctls_out, &user_uffdio_register->ioctls))
 			ret = -EFAULT;
 	}
 out:
-- 
2.17.1