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

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Message-ID: <20121123163320.GA32716@redhat.com>
Date:	Fri, 23 Nov 2012 17:33:20 +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

Hello Amnon,

I am a bit confused,

On 11/23, Amnon Shiloh wrote:
>
> What I discovered now, is that PTRACE_SYSCALL (also PTRACE_SINGLESTEP)
> does not work within the vsyscall page, so I cannot trap the kernel-calls
> there (this is very simple to verify using "gdb" or "strace").

Sure, but we alredy discussed this?

Once again, PTRACE_SYSCALL should work in the NATIVE mode. Obviously it
won't work in EMULATE mode but we can change emulate_vsyscall() to report
TRAP_VSYSCALL or even introduce PTRACE_EVENT_VSYSCALL.

> The necessary patch was already discussed and is very simple.

Do you mean TRAP_VSYSCALL/PTRACE_EVENT_VSYSCALL above or additional
in_gate_area_no_mm() check to allow the hw bp?

> Or, there is an alternative: if only I (the ptracer or the traced process)
> was allowed to munmap the vsyscall page,

It is not possible to unmap it. The kernel (swapper_pg_dir) has this
mapping, not the process. Unlike vdso. IOW, you can only "unmap" it
globally and obviously you can't do this from the userspace.

Oleg.

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