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Date:   Fri, 14 Aug 2020 16:58:23 +0200
From:   Cyril Hrubis <chrubis@suse.cz>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        kernel test robot <rong.a.chen@intel.com>,
        LKML <linux-kernel@vger.kernel.org>, lkp@lists.01.org,
        Thomas Gleixner <tglx@linutronix.de>, ltp@lists.linux.it
Subject: Re: [LTP] [x86/entry] 2bbc68f837: ltp.ptrace08.fail
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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");

So this works actually nicely, even better than the original code.

Any hints on how to select a fixed address in the kernel range as you
pointed out in one of the previous emails? I guess that this would end
up as a per-architecture mess of ifdefs if we wanted to hardcode it.

-- 
Cyril Hrubis
chrubis@suse.cz