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

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Date:	Fri, 18 Mar 2016 18:12:20 +0000
From:	James Morse <james.morse@....com>
To:	Pratyush Anand <panand@...hat.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 Pratyush,

On 18/03/16 14:43, Pratyush Anand wrote:
> On 18/03/2016:02:02:49 PM, James Morse wrote:
>> 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.

Hmmm, I see kernel_enable_single_step() doesn't set it, but setup_singlestep()
in patch 5 could...

There is probably a good reason its never set for a kernel thread, I will have a
look at where else it is used.


> But, we can introduce a
> variant disable_step_kernel and enable_step_kernel, which can be called in
> el1_da. 

What about sp/pc misalignment, or undefined instructions?
Or worse... an irq occurs during your el1_da call (el1_da may re-enable irqs).
el1_irq doesn't know you were careful not to unmask debug exceptions, it blindly
turns them back on.

The problem is the 'single step me' bit is still set, save/restoring it will
save us having to consider every interaction, (and then missing some!).

It would also mean you don't have to disable interrupts while single stepping in
patch 5 (comment above kprobes_save_local_irqflag()).


Thanks,

James