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

[Nadav Amit <namit@vmware.com>]

On Jun 13, 2022, at 3:38 PM, Axel Rasmussen <axelrasmussen@google.com> wrote:

> On Mon, Jun 13, 2022 at 3:29 PM Peter Xu <peterx@redhat.com> 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@google.com> 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.

If this direction is pursued, I think that it would be better to set it as
/proc/[pid]/userfaultfd, which would allow remote monitors (processes) to
hook into userfaultfd of remote processes. I have a patch for that which
extends userfaultfd syscall, but /proc/[pid]/userfaultfd may be cleaner.