linux-kernel - Re: [LTP] [x86/entry] 2bbc68f837: ltp.ptrace08.fail

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Message-ID: <20200812093114.GA13676@yuki.lan>
Date:   Wed, 12 Aug 2020 11:31:14 +0200
From:   Cyril Hrubis <chrubis@...e.cz>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        kernel test robot <rong.a.chen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        Thomas Gleixner <tglx@...utronix.de>, ltp@...ts.linux.it
Subject: Re: [LTP] [x86/entry] 2bbc68f837: ltp.ptrace08.fail

Hi!
> do_debug is a bit of a red herring here.  ptrace should not be able to
> put a breakpoint on a kernel address, period.  I would just pick a
> fixed address that's in the kernel text range or even just in the
> pre-KASLR text range and make sure it gets rejected.  Maybe try a few
> different addresses for good measure.

I've looked at the code and it seems like this would be a bit more
complicated since the breakpoint is set by an accident in a race and the
call still fails. Which is why the test triggers the breakpoint and
causes infinite loop in the kernel...

I guess that we could instead read back the address with
PTRACE_PEEKUSER, so something as:


break_addr = ptrace(PTRACE_PEEKUSER, child_pid,
                    (void *)offsetof(struct user, u_debugreg[0]),
                    NULL);

if (break_addr == kernel_addr)
	tst_res(TFAIL, "ptrace() set break on a kernel address");

-- 
Cyril Hrubis
chrubis@...e.cz