Date: Fri, 18 Mar 2016 20:13:25 +0530
From: Pratyush Anand <panand@redhat.com>
To: James Morse <james.morse@arm.com>
Cc: David Long <dave.long@linaro.org>, Will Deacon <will.deacon@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Sandeepa Prabhu <sandeepa.s.prabhu@gmail.com>,
        William Cohen <wcohen@redhat.com>,
        Steve Capper <steve.capper@linaro.org>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        Marc Zyngier <marc.zyngier@arm.com>,
        Dave P Martin <Dave.Martin@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Robin Murphy <Robin.Murphy@arm.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Jens Wiklander <jens.wiklander@linaro.org>,
        Christoffer Dall <christoffer.dall@linaro.org>,
        Alex =?iso-8859-1?Q?Benn=E9e?= <alex.bennee@linaro.org>,
        Yang Shi <yang.shi@linaro.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        "Suzuki K. Poulose" <suzuki.poulose@arm.com>,
        Kees Cook <keescook@chromium.org>, Zi Shen Lim <zlim.lnx@gmail.com>,
        John Blackwood <john.blackwood@ccur.com>, Feng Kan <fkan@apm.com>,
        Balamurugan Shanmugam <bshanmugam@apm.com>,
        Vladimir Murzin <Vladimir.Murzin@arm.com>,
        Mark Salyzyn <salyzyn@android.com>, Petr Mladek <pmladek@suse.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mark Brown <broonie@kernel.org>
Subject: Re: [PATCH v11 3/9] arm64: add copy_to/from_user to kprobes blacklist
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Hi James,

On 18/03/2016:02:02:49 PM, James Morse wrote:
> Hi Pratyush,
> 
> On 18/03/16 13:29, Pratyush Anand wrote:
> > Probably, I can see why does not it work. So, when we are single stepping an
> > instruction and page fault occurs, we will come to el1_da in entry.S. Here, we
> > do enable_dbg. As soon as we will do this, we will start receiving single step
> > exception after each instruction (not sure, probably for each alternate
> > instruction). Since, there will not be any matching single step handler for
> > these instructions, so we will see warning "Unexpected kernel single-step
> > exception at EL1". 
> > 
> > So, I think, we should 
> > 
> > (1) may be do not enable debug for el1_da, or
> > (2) enable_dbg only when single stepping is not enabled, or
> > (3) or disable single stepping during el1_da execution.
> > 
> > (1) will solve the issue for sure, but not sure if it could be the best choice.
> 
> A variation on (3):
> 
> In kernel/entry.S when entered from EL0 we test for TIF_SINGLESTEP in the
> thread_info flags, and use disable_step_tsk/enable_step_tsk to save/restore the
> single-step state.
> 
> Could we do this regardless of which EL we came from?

Thanks for another idea. I think, we can not do this as it is, because
TIF_SINGLESTEP will not be set for kprobe events. But, we can introduce a
variant disable_step_kernel and enable_step_kernel, which can be called in
el1_da. 

I will write a test case to reproduce the issue without this patch, and then
will do test with a patch based on something like above.

~Pratyush