Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20220601210951.3916598-1-axelrasmussen@google.com>
 <20220601210951.3916598-3-axelrasmussen@google.com> <20220613145540.1c9f7750092911bae1332b92@linux-foundation.org>
 <Yqe6R+XSH+nFc8se@xz-m1.local> <CAJHvVchdmV42qCgO6j=zGBi0DeVcvW1OC88rHUP6V66Fg3CSww@mail.gmail.com>
 <87k09kxi59.fsf@meer.lwn.net>
In-Reply-To: <87k09kxi59.fsf@meer.lwn.net>
From:   Axel Rasmussen <axelrasmussen@google.com>
Date:   Tue, 14 Jun 2022 13:23:52 -0700
Message-ID: <CAJHvVcjAnewRATU3AAZK+TFpNbfiVATfd1tt_ok2k+X+3s3dBA@mail.gmail.com>
Subject: Re: [PATCH v3 2/6] userfaultfd: add /dev/userfaultfd for fine grained
 access control
To:     Jonathan Corbet <corbet@lwn.net>
Cc:     Peter Xu <peterx@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Charan Teja Reddy <charante@codeaurora.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "Dmitry V . Levin" <ldv@altlinux.org>,
        Gleb Fotengauer-Malinovskiy <glebfm@altlinux.org>,
        Hugh Dickins <hughd@google.com>, Jan Kara <jack@suse.cz>,
        Mel Gorman <mgorman@techsingularity.net>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@kernel.org>, Nadav Amit <namit@vmware.com>,
        Shuah Khan <shuah@kernel.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        zhangyi <yi.zhang@huawei.com>, linux-doc@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        Linuxkselftest <linux-kselftest@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Mon, Jun 13, 2022 at 4:23 PM Jonathan Corbet <corbet@lwn.net> wrote:
>
> Axel Rasmussen <axelrasmussen@google.com> writes:
>
> > 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.
>
> I take it there's a reason why this can't be done with a security module
> - either a custom module or a policy in one of the existing modules?
> That sort of access control is just what security modules are supposed
> to be for, after all.
>
> Thanks,
>
> jon

Admittedly I haven't tried proposing a patch, but I suspect there
would be pushback against adding an entirely new LSM just for this
case, similarly to the reasons the CAP_USERFAULTFD approach was
rejected.

For existing LSMs, I think SELinux can be used to restrict access to
syscalls. But then again, it's fairly heavy weight / difficult to
configure, and I suspect migrating production servers which don't use
it today would be a nontrivial undertaking. At least to me it seems
unfortunate to say, there isn't an obvious "safe" way to use
userfaultfd, without enabling + configuring selinux. (That assumes by
"safe" we mean, without granting wider-than necessary access to
userfaultfd, or without granting uffd-using processes more permissions
[root or CAP_SYS_PTRACE] to do their job.) I suspect if we do that
then in practice many? most? users will just either run UFFD programs
as root, or toggle the sysctl to allow unprivileged UFFD usage.