Received-SPF: pass (google.com: domain of linux-kernel+bounces-92886-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45e3:2400::1 as permitted sender) client-ip=2604:1380:45e3:2400::1;
From: "Adrian Ratiu" <adrian.ratiu@collabora.com>
In-Reply-To: <202403051033.9527DD75@keescook>
Content-Type: text/plain; charset="utf-8"
References: <20240301213442.198443-1-adrian.ratiu@collabora.com>
 <20240304-zugute-abtragen-d499556390b3@brauner>
 <202403040943.9545EBE5@keescook>
 <20240305-attentat-robust-b0da8137b7df@brauner>
 <202403050134.784D787337@keescook>
 <20240305-kontakt-ticken-77fc8f02be1d@brauner>
 <202403050211.86A44769@keescook>
 <20240305-brotkrumen-vorbild-9709ce924d25@brauner> <202403051033.9527DD75@keescook>
Date: Tue, 05 Mar 2024 19:34:34 +0000
Cc: "Christian Brauner" <brauner@kernel.org>, linux-fsdevel@vger.kernel.org, kernel@collabora.com, linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, "Guenter Roeck" <groeck@chromium.org>, "Doug Anderson" <dianders@chromium.org>, "Jann Horn" <jannh@google.com>, "Andrew Morton" <akpm@linux-foundation.org>, "Randy Dunlap" <rdunlap@infradead.org>, "Mike Frysinger" <vapier@chromium.org>
To: "Kees Cook" <keescook@chromium.org>
Precedence: bulk
MIME-Version: 1.0
Message-ID: <45d98-65e77400-5-31aa8000@248840925>
Subject: =?utf-8?q?Re=3A?= [PATCH v2] =?utf-8?q?proc=3A?= allow restricting 
 /proc/pid/mem writes
User-Agent: SOGoMail 5.10.0
Content-Transfer-Encoding: quoted-printable

On Tuesday, March 05, 2024 20:37 EET, Kees Cook <keescook@chromium.org>=
 wrote:

> On Tue, Mar 05, 2024 at 11:32:04AM +0100, Christian Brauner wrote:
> > On Tue, Mar 05, 2024 at 02:12:26AM -0800, Kees Cook wrote:
> > > On Tue, Mar 05, 2024 at 10:58:25AM +0100, Christian Brauner wrote=
:
> > > > Since the write handler for /proc/<pid>/mem does raise FOLL=5FF=
ORCE
> > > > unconditionally it likely would implicitly. But I'm not familia=
r enough
> > > > with FOLL=5FFORCE to say for sure.
> > >=20
> > > I should phrase the question better. :) Is the supervisor writing=
 into
> > > read-only regions of the child process?
> >=20
> > Hm... I suspect we don't. Let's take two concrete examples so you c=
an
> > tell me.
> >=20
> > Incus intercepts the sysinfo() syscall. It prepares a struct sysinf=
o
> > with cgroup aware values for the supervised process and then does:
> >=20
> > unix.Pwrite(siov.memFd, &sysinfo, sizeof(struct sysinfo), seccomp=5F=
data.args[0]))
> >=20
> > It also intercepts some bpf system calls attaching bpf programs for=
 the
> > caller. If that fails we update the log buffer for the supervised
> > process:
> >=20
> > union bpf=5Fattr attr =3D {}, new=5Fattr =3D {};
> >=20
> > // read struct bpf=5Fattr from mem=5Ffd
> > ret =3D pread(mem=5Ffd, &attr, attr=5Flen, req->data.args[1]);
> > if (ret < 0)
> >         return -errno;
> >=20
> > // Do stuff with attr. Stuff fails. Update log buffer for supervise=
d process:
> > if ((new=5Fattr.log=5Fsize) > 0 && (pwrite(mem=5Ffd, new=5Fattr.log=
=5Fbuf, new=5Fattr.log=5Fsize, attr.log=5Fbuf) !=3D new=5Fattr.log=5Fsi=
ze))
>=20
> This is almost certainly in writable memory (either stack or .data).

Mostly yes, but we can't be certain where it comes from, because
SECCOMP=5FIOCTL=5FNOTIF=5FRECV passes any addresses set by the
caller to the supervisor process.

It is a kind of "implementation defined" behavior, just like we
can't predict what the supervisor will do with the caller mem :)

>=20
> > But I'm not sure if there are other use-cases that would require th=
is.
>=20
> Maybe this option needs to be per-process (like no=5Fnew=5Fprivs), an=
d with
> a few access levels:
>=20
> - as things are now
> - no FOLL=5FFORCE unless by ptracer
> - no writes unless by ptracer
> - no FOLL=5FFORCE ever
> - no writes ever
> - no reads unless by ptracer
> - no reads ever
>=20
> Which feels more like 3 toggles: read, write, FOLL=5FFORCE. Each set =
to
> "DAC", "ptracer", and "none"?

I really like this approach because it provides a  mechanism
with maximum flexibility without imposing a specific policy.

What does DAC mean in this context? My mind jumps to
Digital to Analog Converter :)

Shall I give it a try in v3?

>=20
> --=20
> Kees Cook