userfaultfd: open userfaultfds with O_RDONLY

Since userfaultfd doesn't implement a write operation, it is more
appropriate to open it read-only.

When userfaultfds are opened read-write like it is now, and such fd is
passed from one process to another, SELinux will check both read and
write permissions for the target process, even though it can't actually
do any write operation on the fd later.

Inspired by the following bug report, which has hit the SELinux scenario
described above:
https://bugzilla.redhat.com/show_bug.cgi?id=1974559Reported-by: Robert O'Callahan <roc@ocallahan.org>
Fixes: 86039bd3 ("userfaultfd: add new syscall to provide memory externalization")
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Acked-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Paul Moore <paul@paul-moore.com>

fs/userfaultfd.c

@@ -991,7 +991,7 @@ static int resolve_userfault_fork(struct userfaultfd_ctx *new,
 	int fd;
 
 	fd = anon_inode_getfd_secure("[userfaultfd]", &userfaultfd_fops, new,
-			O_RDWR | (new->flags & UFFD_SHARED_FCNTL_FLAGS), inode);
+			O_RDONLY | (new->flags & UFFD_SHARED_FCNTL_FLAGS), inode);
 	if (fd < 0)
 		return fd;
 
@@ -2090,7 +2090,7 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
 	mmgrab(ctx->mm);
 
 	fd = anon_inode_getfd_secure("[userfaultfd]", &userfaultfd_fops, ctx,
-			O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS), NULL);
+			O_RDONLY | (flags & UFFD_SHARED_FCNTL_FLAGS), NULL);
 	if (fd < 0) {
 		mmdrop(ctx->mm);
 		kmem_cache_free(userfaultfd_ctx_cachep, ctx);