Re: [PATCH v3 2/6] userfaultfd: add /dev/userfaultfd for fine grained access control

[Axel Rasmussen]

On Mon, Jun 13, 2022 at 3:29 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> On Mon, Jun 13, 2022 at 02:55:40PM -0700, Andrew Morton wrote:
> > On Wed,  1 Jun 2022 14:09:47 -0700 Axel Rasmussen <axelrasmussen@xxxxxxxxxx> wrote:
> >
> > > To achieve this, add a /dev/userfaultfd misc device. This device
> > > provides an alternative to the userfaultfd(2) syscall for the creation
> > > of new userfaultfds. The idea is, any userfaultfds created this way will
> > > be able to handle kernel faults, without the caller having any special
> > > capabilities. Access to this mechanism is instead restricted using e.g.
> > > standard filesystem permissions.
> >
> > The use of a /dev node isn't pretty.  Why can't this be done by
> > tweaking sys_userfaultfd() or by adding a sys_userfaultfd2()?

I think for any approach involving syscalls, we need to be able to
control access to who can call a syscall. Maybe there's another way
I'm not aware of, but I think today the only mechanism to do this is
capabilities. I proposed adding a CAP_USERFAULTFD for this purpose,
but that approach was rejected [1]. So, I'm not sure of another way
besides using a device node.

One thing that could potentially make this cleaner is, as one LWN
commenter pointed out, we could have open() on /dev/userfaultfd just
return a new userfaultfd directly, instead of this multi-step process
of open /dev/userfaultfd, NEW ioctl, then you get a userfaultfd. When
I wrote this originally it wasn't clear to me how to get that to
happen - open() doesn't directly return the result of our custom open
function pointer, as far as I can tell - but it could be investigated.

[1]: https://lore.kernel.org/lkml/686276b9-4530-2045-6bd8-170e5943abe4@xxxxxxxxxxxxxxxx/T/

> >
> > Peter, will you be completing review of this patchset?
>
> Sorry to not have reviewed it proactively..
>
> I think it's because I never had a good picture/understanding of what
> should be the best security model for uffd, meanwhile I am (it seems) just
> seeing more and more ways to "provide a safer uffd" by different people
> using different ways.. and I never had time (and probably capability too..)
> to figure out the correct approach if not to accept all options provided.

Agreed, what we have right now is a bit of a mess of different
approaches. I think the reason for this is, there is no "perfect" way
to control access to features like this, so what we now have is
several different approaches with different tradeoffs.

>From my perspective, the existing controls were simpler to implement,
but are not ideal because they require us to grant access to UFFD
*plus more stuff too*.

The approach I've proposed is the most granular, so it doesn't require
adding any extra permissions. But, I agree the interface is sort of
overcomplicated. :/ But, from my perspective, security in shared Cloud
computing environments where UFFD is used for live migration is
critical, so I prefer this tradeoff - I'll put up with a slightly
messier interface, if the gain is a very minimal set of privileges.

>
> I think I'll just assume the whole thing is acked already from you
> generally, then I'll read at least the implementation before the end of
> tomorrow.
>
> Thanks,
>
> --
> Peter Xu
>