Re: [PATCH RFC] seccomp: Implement syscall isolation based on memory areas

[Andy Lutomirski <luto@amacapital.net>]

> On May 30, 2020, at 5:26 PM, Gabriel Krisman Bertazi <krisman@collabora.com> wrote:
> 
> 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?

I’m suggesting that the kernel learn how to help you, maybe like this:

prctl(PR_SET_SYSCALL_THUNK, target, address_of_unredirected_syscall, 0, 0, 0, 0);

This would be inherited on clone/fork and cleared on execve.

> 
>> 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