linux-kernel - Re: [PATCH RFC v1] arm64: Handle traps from accessing CNTVCT/CNTFRQ for CONFIG

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Message-ID: <86r2qpec32.wl-marc.zyngier@arm.com>
Date:   Tue, 16 Jan 2018 21:19:13 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Nicolin Chen <nicoleotsuka@...il.com>
Cc:     mark.rutland@....com, catalin.marinas@....com, will.deacon@....com,
        oleg@...hat.com, cdall@...aro.org, tbaicar@...eaurora.org,
        julien.thierry@....com, Dave.Martin@....com, robin.murphy@....com,
        james.morse@....com, ard.biesheuvel@...aro.org,
        xiexiuqi@...wei.com, mingo@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v1] arm64: Handle traps from accessing CNTVCT/CNTFRQ for CONFIG_COMPAT

On Tue, 16 Jan 2018 20:32:19 +0000,
Nicolin Chen wrote:
> 
> Hello Marc,
> 
> On Thu, Jan 11, 2018 at 08:51:37AM +0000, Marc Zyngier wrote:
> > > [ I also added cntfrq here for safety as theoretically it could
> > >   trigger the trap as well. However, my another test case (with
> > >   mrc insturction) doesn't seem to trigger a trap. So I would
> > >   drop it in the next version if someone can confirm it's not
> > >   required. Thanks -- Nicolin ]
> > 
> > See my previous series on this very subject[1] as well as Will's reply.
> 
> Thanks for the background.
> 
> > > -	for (hook = sys64_hooks; hook->handler; hook++)
> > > +	for (; hook && hook->handler; hook++)
> > >  		if ((hook->esr_mask & esr) == hook->esr_val) {
> > >  			hook->handler(esr, regs);
> > >  			return;
> > > 
> > 
> > Also, this code is fairly broken in its handling of conditional
> > instructions.
> 
> I understand that it should take care of the condition field as
> a general instruction handler. Just for curiosity: If we confine
> the topic to read access of CNTVCT/CNTFRQ, what'd be the penalty
> by ignoring the condition field and executing it anyway?

Do you mean, apart from severely corrupting userspace execution?
That's a rhetorical question, right?

	M.
-- 
Jazz is not dead, it just smell funny.