linux-kernel - Re: [PATCH v11 3/9] arm64: add copy_to/from

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Message-ID: <20160318144325.GB29225@dhcppc6.redhat.com>
Date:	Fri, 18 Mar 2016 20:13:25 +0530
From:	Pratyush Anand <panand@...hat.com>
To:	James Morse <james.morse@....com>
Cc:	David Long <dave.long@...aro.org>,
	Will Deacon <will.deacon@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Sandeepa Prabhu <sandeepa.s.prabhu@...il.com>,
	William Cohen <wcohen@...hat.com>,
	Steve Capper <steve.capper@...aro.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Marc Zyngier <marc.zyngier@....com>,
	Dave P Martin <Dave.Martin@....com>,
	Mark Rutland <mark.rutland@....com>,
	Robin Murphy <Robin.Murphy@....com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Jens Wiklander <jens.wiklander@...aro.org>,
	Christoffer Dall <christoffer.dall@...aro.org>,
	Alex Bennée <alex.bennee@...aro.org>,
	Yang Shi <yang.shi@...aro.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	"Suzuki K. Poulose" <suzuki.poulose@....com>,
	Kees Cook <keescook@...omium.org>,
	Zi Shen Lim <zlim.lnx@...il.com>,
	John Blackwood <john.blackwood@...r.com>,
	Feng Kan <fkan@....com>,
	Balamurugan Shanmugam <bshanmugam@....com>,
	Vladimir Murzin <Vladimir.Murzin@....com>,
	Mark Salyzyn <salyzyn@...roid.com>,
	Petr Mladek <pmladek@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH v11 3/9] arm64: add copy_to/from_user to kprobes blacklist

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