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=1974559

Reported-by: Robert O'Callahan <redacted>
Fixes: 86039bd3b4e6 ("userfaultfd: add new syscall to provide memory externalization")
Signed-off-by: Ondrej Mosnacek <redacted>
---

I marked this as RFC, because I'm not sure if this has any unwanted side
effects. I only ran this patch through selinux-testsuite, which has a
simple userfaultfd subtest, and a reproducer from the Bugzilla report.

Please tell me whether this makes sense and/or if it passes any
userfaultfd tests you guys might have.

 fs/userfaultfd.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 14f92285d04f..24e14c36068f 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c

@@ -986,7 +986,7 @@ 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;