Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
From:   Andy Lutomirski <luto@amacapital.net>
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH RFC] seccomp: Implement syscall isolation based on memory areas
Date:   Mon, 1 Jun 2020 06:59:26 -0700
Message-Id: <3691744C-F4BC-49C6-9450-52E31DD14A92@amacapital.net>
References: <CAFUUQHO57YpWJv6oGmxXsOBxCzRG1ti_Q1QBDw8FU+4vBOPgBw@mail.gmail.com>
Cc:     krisman@collabora.com, gofmanp@gmail.com, hpa@zytor.com,
        keescook@chromium.org, kernel@collabora.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        tglx@linutronix.de, wad@chromium.org
In-Reply-To: <CAFUUQHO57YpWJv6oGmxXsOBxCzRG1ti_Q1QBDw8FU+4vBOPgBw@mail.gmail.com>
To:     Billy Laws <blaws05@gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Jun 1, 2020, at 2:23 AM, Billy Laws <blaws05@gmail.com> wrote:
>=20
> =EF=BB=BF
>>=20
>> On May 30, 2020, at 5:26 PM, Gabriel Krisman Bertazi <krisman@collabora.c=
om> wrote:
>>=20
>> =EF=BB=BFAndy Lutomirski <luto@amacapital.net> writes:
>>=20
>>>>>> On May 29, 2020, at 11:00 PM, Gabriel Krisman Bertazi <krisman@collab=
ora.com> wrote:
>>>>>=20
>>>>> =EF=BB=BFModern Windows applications are executing system call instruc=
tions
>>>>> directly from the application's code without going through the WinAPI.=

>>>>> This breaks Wine emulation, because it doesn't have a chance to
>>>>> intercept and emulate these syscalls before they are submitted to Linu=
x.
>>>>>=20
>>>>> In addition, we cannot simply trap every system call of the applicatio=
n
>>>>> to userspace using PTRACE_SYSEMU, because performance would suffer,
>>>>> since our main use case is to run Windows games over Linux.  Therefore=
,
>>>>> we need some in-kernel filtering to decide whether the syscall was
>>>>> issued by the wine code or by the windows application.
>>>=20
>>> Do you really need in-kernel filtering?  What if you could have
>>> efficient userspace filtering instead?  That is, set something up so
>>> that all syscalls, except those from a special address, are translated
>>> to CALL thunk where the thunk is configured per task.  Then the thunk
>>> can do whatever emulation is needed.
>>=20
>> Hi,
>>=20
>> I suggested something similar to my customer, by using
>> libsyscall-intercept.  The idea would be overwritting the syscall
>> instruction with a call to the entry point.  I'm not a specialist on the
>> specifics of Windows games, (cc'ed Paul Gofman, who can provide more
>> details on that side), but as far as I understand, the reason why that
>> is not feasible is that the anti-cheat protection in games will abort
>> execution if the binary region was modified either on-disk or in-memory.
>>=20
>> Is there some mechanism to do that without modiyfing the application?
>=20
> Hi,
>=20
> I work on an emulator for the Nintendo Switch that uses a similar techniqu=
e,
> in our testing it works very well and is much more performant than even
> PTRACE_SYSEMU.
>=20
> To work around DRM reading the memory contents I think mprotect could
> be used, after patching the syscall a copy of the original code could be
> kept somewhere in memory and the patched region mapped --X.
> With this, any time the DRM attempts to read to the patched region and
> perform integrity checks it will cause a segfault and a branch to the
> signal handler. This handler can then return the contents of the original,=

> unpatched region to satisfy them checks.
>=20
> Are memory contents checked by DRM solutions too often for this to be
> performant?

A bigger issue is that hardware support for =E2=80=94X is quite spotty. Ther=
e is no x86 CPU that can do it cleanly in a bare metal setup, and client CPU=
s that can do it at all without hypervisor help may be nonexistent. I don=E2=
=80=99t know if the ARM situation is much better.

> --
> Billy Laws
>>=20
>>> Getting the details and especially the interaction with any seccomp
>>> filters that may be installed right could be tricky, but the performance=

>>> should be decent, at least on non-PTI systems.
>>>=20
>>> (If we go this route, I suspect that the correct interaction with
>>> seccomp is that this type of redirection takes precedence over seccomp
>>> and seccomp filters are not invoked for redirected syscalls. After all,
>>> a redirected syscall is, functionally, not a syscall at all.)
>>>=20
>>=20
>>=20
>> --
>> Gabriel Krisman Bertazi