linux-kernel - Re: [PATCH v5 3/8] arm: fixmap: implement __set

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Date:	Wed, 10 Sep 2014 18:51:26 +0100
From:	Will Deacon <will.deacon@....com>
To:	Kees Cook <keescook@...omium.org>
Cc:	Rabin Vincent <rabin@....in>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Laura Abbott <lauraa@...eaurora.org>,
	Rob Herring <robh@...nel.org>,
	Leif Lindholm <leif.lindholm@...aro.org>,
	"msalter@...hat.com" <msalter@...hat.com>,
	Liu hua <sdu.liu@...wei.com>,
	Nikolay Borisov <Nikolay.Borisov@....com>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Doug Anderson <dianders@...gle.com>,
	Jason Wessel <jason.wessel@...driver.com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v5 3/8] arm: fixmap: implement __set_fixmap()

Hi Kees,

On Tue, Sep 09, 2014 at 03:33:11PM +0100, Kees Cook wrote:
> On Tue, Sep 9, 2014 at 5:38 AM, Will Deacon <will.deacon@....com> wrote:
> > On Mon, Sep 08, 2014 at 11:40:43PM +0100, Kees Cook wrote:
> >> Ah, so it was, yes! Will, which version of this logic would you prefer?
> >
> > I still don't think we're solving the general problem here -- we're actually
> > just making the ftrace case work. It is perfectly possible for another CPU
> > to undergo a TLB miss and refill whilst the page table is being modified by
> > the CPU with preemption disabled. In this case, a local tlb flush won't
> > invalidate that entry on the other core, and we have no way of knowing when
> > the original permissions are actually observed across the system.
> 
> The fixmap is used by anything doing patching _except_ ftrace,
> actually. It's used by jump labels, kprobes, and kgdb. This code is
> the general case. Access to set_fixmap is done via the kernel patching
> interface: patch_text().
> 
> Right now, the patch_text interface checks cache_ops_need_broadcast(),
> and conditionally runs under stop_machine(). We could make this
> unconditional, and we'll avoid any problem with TLB misses on another
> CPU.

Yes, it we always use stop_machine, that solves the TLB broadcast problem
and we could do that if CONFIG_ARM_ERRATA_798181 is set.

> > So I think we need to figure out a way to invalidate the TLB properly. What
> > do architectures that use IPIs for TLB broadcasting do (x86, some powerpc,
> > mips, ...)? They must have exactly the same problem.
> 
> I don't think this should be done at the set_fixmap level, as it is
> more a primitive. I think making sure patch_text() always works would
> be best. What do you think of using an unconditional stop_machine()
> instead?

Why not move the TLB invalidation into patch_text, then we can do
stop_machine if IS_ENABLED(CONFIG_ARM_ERRATA_798181) ||
tlb_ops_need_broadcast()?

Then that just leaves ftrace.

Will
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