[RFC PATCH RESEND] userfaultfd: open userfaultfds with O_RDONLY

[Paul Moore]

On Fri, Jul 8, 2022 at 5:35 AM 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>
> ---
>
> Resending as the last submission was ignored for over a year...
>
> https://lore.kernel.org/lkml/20210624152515.1844133-1-omosnace@redhat.com/T/
>
> 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(-)

VFS folks, any objection to this patch?  It seems reasonable to me and
I'd really prefer this to go in via the vfs tree, but I'm not above
merging this via the lsm/next tree to get someone in vfs land to pay
attention to this ...

> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index e943370107d0..8ccf00be63e1 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -989,7 +989,7 @@ 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);
> --
> 2.36.1

-- 
paul-moore.com