Re: [PATCH 3/7] Add a UFFD_SECURE flag to the userfaultfd API.

[Daniel Colascione <dancol@google.com>]

On Sat, Oct 12, 2019 at 6:14 PM Andy Lutomirski <luto@kernel.org> wrote:
>
..
>
> > But maybe we can go further: let's separate authentication and
> > authorization, as we do in other LSM hooks. Let's split my
> > inode_init_security_anon into two hooks, inode_init_security_anon and
> > inode_create_anon. We'd define the former to just initialize the file
> > object's security information --- in the SELinux case, figuring out
> > its class and SID --- and define the latter to answer the yes/no
> > question of whether a particular anonymous inode creation should be
> > allowed. Normally, anon_inode_getfile2() would just call both hooks.
> > We'd add another anon_inode_getfd flag, ANON_INODE_SKIP_AUTHORIZATION
> > or something, that would tell anon_inode_getfile2() to skip calling
> > the authorization hook, effectively making the creation always
> > succeed. We can then make the UFFD code pass
> > ANON_INODE_SKIP_AUTHORIZATION when it's creating a file object in the
> > fork child while creating UFFD_EVENT_FORK messages.
>
> That sounds like an improvement.  Or maybe just teach SELinux that
> this particular fd creation is actually making an anon_inode that is a
> child of an existing anon inode and that the context should be copied
> or whatever SELinux wants to do.  Like this, maybe:
>
> static int resolve_userfault_fork(struct userfaultfd_ctx *ctx,
>                                   struct userfaultfd_ctx *new,
>                                   struct uffd_msg *msg)
> {
>         int fd;
>
> Change this:
>
>         fd = anon_inode_getfd("[userfaultfd]", &userfaultfd_fops, new,
>                               O_RDWR | (new->flags & UFFD_SHARED_FCNTL_FLAGS));
>
> to something like:
>
>       fd = anon_inode_make_child_fd(..., ctx->inode, ...);
>
> where ctx->inode is the one context's inode.

Yeah. I figured we could just add a special-purpose hook for this
case. Having a special hook for this one case feels ugly though, and
at copy_mm time, we don't have a PID for the new child yet --- I don't
know whether LSMs would care about that. But maybe this is one of
those "doctor, it hurts when I do this!" situations and this child
process difficulty is just a hint that some other design might work
better.

> Now that you've pointed this mechanism out, it is utterly and
> completely broken and should be removed from the kernel outright or at
> least severely restricted.  A .read implementation MUST NOT ACT ON THE
> CALLING TASK.  Ever.  Just imagine the effect of passing a userfaultfd
> as stdin to a setuid program.
>
> So I think the right solution might be to attempt to *remove*
> UFFD_EVENT_FORK.  Maybe the solution is to say that, unless the
> creator of a userfaultfd() has global CAP_SYS_ADMIN, then it cannot
> use UFFD_FEATURE_EVENT_FORK) and print a warning (once) when
> UFFD_FEATURE_EVENT_FORK is allowed.  And, after some suitable
> deprecation period, just remove it.  If it's genuinely useful, it
> needs an entirely new API based on ioctl() or a syscall.  Or even
> recvmsg() :)

IMHO, userfaultfd should have been a datagram socket from the start.
As you point out, it's a good fit for the UFFD protocol, which
involves FD passing and a fixed message size.

> And UFFD_SECURE should just become automatic, since you don't have a
> problem any more. :-p

Agreed. I'll wait to hear what everyone else has to say.