Re: [PATCH v4 2/5] userfaultfd: add /dev/userfaultfd for fine grained access control

[Axel Rasmussen]

On Tue, Jul 19, 2022 at 3:32 PM Nadav Amit <namit@xxxxxxxxxx> wrote:
>
> On Jul 19, 2022, at 12:56 PM, Axel Rasmussen <axelrasmussen@xxxxxxxxxx> wrote:
>
> > Historically, it has been shown that intercepting kernel faults with
> > userfaultfd (thereby forcing the kernel to wait for an arbitrary amount
> > of time) can be exploited, or at least can make some kinds of exploits
> > easier. So, in 37cd0575b8 "userfaultfd: add UFFD_USER_MODE_ONLY" we
> > changed things so, in order for kernel faults to be handled by
> > userfaultfd, either the process needs CAP_SYS_PTRACE, or this sysctl
> > must be configured so that any unprivileged user can do it.
> >
> > In a typical implementation of a hypervisor with live migration (take
> > QEMU/KVM as one such example), we do indeed need to be able to handle
> > kernel faults. But, both options above are less than ideal:
> >
> > - Toggling the sysctl increases attack surface by allowing any
> >  unprivileged user to do it.
> >
> > - Granting the live migration process CAP_SYS_PTRACE gives it this
> >  ability, but *also* the ability to "observe and control the
> >  execution of another process [...], and examine and change [its]
> >  memory and registers" (from ptrace(2)). This isn't something we need
> >  or want to be able to do, so granting this permission violates the
> >  "principle of least privilege".
> >
> > This is all a long winded way to say: we want a more fine-grained way to
> > grant access to userfaultfd, without granting other additional
> > permissions at the same time.
> >
> > 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.
>
> Are there any other “devices" that when opened by different processes
> provide such isolated interfaces in each process? I.e., devices that if you
> read from them in different processes you get completely unrelated data?
> (putting aside namespaces).
>
> It all sounds so wrong to me, that I am going to try again to pushback
> (sorry).

No need to be sorry. :)

>
> From a semantic point of view - userfaultfd is process specific. It is
> therefore similar to /proc/[pid]/mem (or /proc/[pid]/pagemap and so on).
>
> So why can’t we put it there? I saw that you argued against it in your
> cover-letter, and I think that your argument is you would need
> CAP_SYS_PTRACE if you want to access userfaultfd of other processes. But
> this is EXACTLY the way opening /proc/[pid]/mem is performed - see
> proc_mem_open().
>
> So instead of having some strange device that behaves differently in the
> context of each process, you can just have /proc/[pid]/userfaultfd and then
> use mm_access() to check if you have permissions to access userfaultfd (just
> like proc_mem_open() does). This would be more intuitive for users as it is
> similar to other /proc/[pid]/X, and would cover both local and remote
> use-cases.

Ah, so actually I find this argument much more compelling.

I don't find it persuasive that we should put it in /proc for the
purpose of supporting cross-process memory manipulation, because I
think the syscall works better for that, and in that case we don't
mind depending on CAP_SYS_PTRACE.

But, what you've argued here I do find persuasive. :) You are right, I
can't think of any other example of a device node in /dev that works
like this, where it is completely independent on a per-process basis.
The closest I could come up with was /dev/zero or /dev/null or
similar. You won't affect any other process by touching these, but I
don't think these are good examples.

I'll send a v5 which does this. I do worry that cross-process support
is probably complex to get right, so I might leave that out and only
allow a process to open its own device for now.

>