linux-kernel - Re: arch_check_bp_in_kernelspace: fix the range check

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Message-ID: <20121125225533.GA24905@redhat.com>
Date:	Sun, 25 Nov 2012 23:55:33 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Amnon Shiloh <u3557@...o.sublimeip.com>
Cc:	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org
Subject: Re: arch_check_bp_in_kernelspace: fix the range check

On 11/25, Amnon Shiloh wrote:
>
> 2) I was then told (in my own words): "oh, don't worry, the vsyscall page
>    has now been minimized, all it contains now is *real* system calls,
>    and it always calls them".

Not sure where did you get this idea ;) From the very beginning you were
told that EMULATE mode doesn't do this.

The NATIVE mode should be fine, yes.

> 6) I just suggested an alternative: to have the whole vsyscall page
>    removed on a per-process basis. I accept your reply that this is
>    not possible.

Yes, this is not possible.

> 7) I suggested a third alternative: to have the vsyscall page be
>    unexecutable on a per-process basis,

Like above, this is simply not possible. And at the same time the
vsyscall page is already unexecutable in EMULATE mode, but globally.

> 8) Any solution that allows a ptracer to prevent its traced process
>    from entering the vsyscall page and execute there system-calls
>    unchecked (thus in effect escape its jailer), would do for me.

Well. I am even more confused... probably this was already discussed
and I missed this, but.

Why do you need to _prevent_, say, sys_gettimeofday()? Why we can't
change emulate_vsyscall() to respect PTRACE_SYSCALL and report
TRAP_VSYSCALL or PTRACE_EVENT_VSYSCALL as I tried to suggest in
http://marc.info/?l=linux-kernel&m=135343635523715 ?

You previously replied that this can not work. Now that you see that
this _can_ work, could you please explain why this is not enough?

Oleg.

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