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From:   Gabriel Krisman Bertazi <krisman@collabora.com>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        kernel@collabora.com, Thomas Gleixner <tglx@linutronix.de>,
        Kees Cook <keescook@chromium.org>,
        Will Drewry <wad@chromium.org>,
        "H . Peter Anvin" <hpa@zytor.com>, Paul Gofman <gofmanp@gmail.com>
Subject: Re: [PATCH RFC] seccomp: Implement syscall isolation based on memory areas
Organization: Collabora
References: <20200530055953.817666-1-krisman@collabora.com>
        <AF65147C-15DB-4BA4-A08B-55676B489BA5@amacapital.net>
Date:   Sat, 30 May 2020 20:26:17 -0400
In-Reply-To: <AF65147C-15DB-4BA4-A08B-55676B489BA5@amacapital.net> (Andy
        Lutomirski's message of "Sat, 30 May 2020 15:09:47 -0700")
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Andy Lutomirski <luto@amacapital.net> writes:

>> On May 29, 2020, at 11:00 PM, Gabriel Krisman Bertazi <krisman@collabora.com> wrote:
>> 
>> ﻿Modern Windows applications are executing system call instructions
>> 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 Linux.
>> 
>> In addition, we cannot simply trap every system call of the application
>> 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.
>
> 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.

Hi,

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.

Is there some mechanism to do that without modiyfing the application?

> 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.
>
> (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.)
>


-- 
Gabriel Krisman Bertazi