[RFC PATCH] userfaultfd: open userfaultfds with O_RDONLY

[Ondrej Mosnacek]

On Thu, Jun 24, 2021 at 5:25 PM Ondrej Mosnacek <omosnace at redhat.com> wrote:
>
> 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 <roc at ocallahan.org>
> Fixes: 86039bd3b4e6 ("userfaultfd: add new syscall to provide memory externalization")
> Signed-off-by: Ondrej Mosnacek <omosnace at redhat.com>
> ---
>
> 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;
>
> @@ -2088,7 +2088,7 @@ 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);
> --
> 2.31.1

Ping? Any comments on this patch?

-- 
Ondrej Mosnacek
Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.