Re: [PATCH v4 0/5] userfaultfd: add /dev/userfaultfd for fine grained access control

[Nadav Amit]

On Jul 20, 2022, at 4:04 PM, Axel Rasmussen <axelrasmussen@xxxxxxxxxx> wrote:

> ⚠ External Email
> 
> On Wed, Jul 20, 2022 at 3:16 PM Schaufler, Casey
> <casey.schaufler@xxxxxxxxx> wrote:
>>> -----Original Message-----
>>> From: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
>>> Sent: Tuesday, July 19, 2022 12:56 PM
>>> To: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>; Andrew Morton
>>> <akpm@xxxxxxxxxxxxxxxxxxxx>; Dave Hansen
>>> <dave.hansen@xxxxxxxxxxxxxxx>; Dmitry V . Levin <ldv@xxxxxxxxxxxx>; Gleb
>>> Fotengauer-Malinovskiy <glebfm@xxxxxxxxxxxx>; Hugh Dickins
>>> <hughd@xxxxxxxxxx>; Jan Kara <jack@xxxxxxx>; Jonathan Corbet
>>> <corbet@xxxxxxx>; Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>; Mike
>>> Kravetz <mike.kravetz@xxxxxxxxxx>; Mike Rapoport <rppt@xxxxxxxxxx>;
>>> Amit, Nadav <namit@xxxxxxxxxx>; Peter Xu <peterx@xxxxxxxxxx>;
>>> Shuah Khan <shuah@xxxxxxxxxx>; Suren Baghdasaryan
>>> <surenb@xxxxxxxxxx>; Vlastimil Babka <vbabka@xxxxxxx>; zhangyi
>>> <yi.zhang@xxxxxxxxxx>
>>> Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>; linux-
>>> doc@xxxxxxxxxxxxxxx; linux-fsdevel@xxxxxxxxxxxxxxx; linux-
>>> kernel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx; linux-
>>> kselftest@xxxxxxxxxxxxxxx
>>> Subject: [PATCH v4 0/5] userfaultfd: add /dev/userfaultfd for fine grained
>>> access control
>> 
>> I assume that leaving the LSM mailing list off of the CC is purely
>> accidental. Please, please include us in the next round.
> 
> Honestly it just hadn't occurred to me, but I'm more than happy to CC
> it on future revisions.
> 
>>> This series is based on torvalds/master.
>>> 
>>> The series is split up like so:
>>> - Patch 1 is a simple fixup which we should take in any case (even by itself).
>>> - Patches 2-6 add the feature, configurable selftest support, and docs.
>>> 
>>> Why not ...?
>>> ============
>>> 
>>> - Why not /proc/[pid]/userfaultfd? The proposed use case for this is for one
>>> process to open a userfaultfd which can intercept another process' page
>>> faults. This seems to me like exactly what CAP_SYS_PTRACE is for, though,
>>> so I
>>> think this use case can simply use a syscall without the powers
>>> CAP_SYS_PTRACE
>>> grants being "too much".
>>> 
>>> - Why not use a syscall? Access to syscalls is generally controlled by
>>> capabilities. We don't have a capability which is used for userfaultfd access
>>> without also granting more / other permissions as well, and adding a new
>>> capability was rejected [1].
>>> 
>>> - It's possible a LSM could be used to control access instead. I suspect
>>> adding a brand new one just for this would be rejected,
>> 
>> You won't know if you don't ask.
> 
> Fair enough - I wonder if MM folks (Andrew, Peter, Nadav especially)
> would find that approach more palatable than /proc/[pid]/userfaultfd?
> Would it make sense from our perspective to propose a userfaultfd- or
> MM-specific LSM for controlling access to certain features?
> 
> I remember +Andrea saying Red Hat was also interested in some kind of
> access control mechanism like this. Would one or the other approach be
> more convenient for you?

To reiterate my position - I think that /proc/[pid]/userfaultfd is very
natural and can be easily extended to support cross-process access of
userfaultfd. The necessary access controls are simple in any case. For
cross-process access, they are similar to those that are used for other
/proc/[pid]/X such as pagemap.

I have little experience with LSM and I do not know how real deployments use
them. If they are easier to deploy and people prefer them over some
pseudo-file, I cannot argue against them.