Re: arch_check_bp_in_kernelspace: fix the range check

From: Oleg Nesterov <oleg@redhat.com>
To: Amnon Shiloh <u3557@miso.sublimeip.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>,
	Pavel Emelyanov <xemul@parallels.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	linux-kernel@vger.kernel.org
Subject: Re: arch_check_bp_in_kernelspace: fix the range check
Date: Fri, 23 Nov 2012 17:33:20 +0100	[thread overview]
Message-ID: <20121123163320.GA32716@redhat.com> (raw)
In-Reply-To: <20121123091453.016B0592076@miso.sublimeip.com>

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.