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

[Nadav Amit <namit@vmware.com>]

On Jun 14, 2022, at 5:55 PM, Axel Rasmussen <axelrasmussen@google.com> wrote:

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> On Mon, Jun 13, 2022 at 5:10 PM Nadav Amit <namit@vmware.com> wrote:
>> 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.
> 
> Hmm, one thing I'm unsure about -
> 
> If a process is able to control another process' memory like this,
> then this seems like exactly what CAP_SYS_PTRACE is intended to deal
> with, right? So I'm not sure this case is directly related to the one
> I'm trying to address.
> 
> This also seems distinct to me versus the existing way you'd do this,
> which is open a userfaultfd and register a shared memory region, and
> then fork(). Now you can control your child's memory with userfaultfd.
> But, attaching to some other, previously-unrelated process with
> /proc/[pid]/userfaultfd seems like a clear case for CAP_SYS_PTRACE.

I agree about CAP_SYS_PTRACE. I just know that if the /dev approach is
taken, there would be even more pushback for userfaultfd2.

Whatever.