linux-kernel - Re: [PATCH] arm64: cpufeature: Trim feature reporting and include PAN emulation

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Date:   Wed, 21 Feb 2018 14:39:55 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Dave Martin <Dave.Martin@....com>
Cc:     Kees Cook <keescook@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm64: cpufeature: Trim feature reporting and include
 PAN emulation

On Wed, Feb 21, 2018 at 11:18:27AM +0000, Dave Martin wrote:
> On Tue, Feb 20, 2018 at 02:46:24PM -0800, Kees Cook wrote:
> > The PAN emulation notification was only happening for non-boot CPUs
> > if CPU capabilities had already been configured. This seems to be the
> > wrong place, as it's system-wide and isn't attached to capabilities,
> > so its reporting didn't normally happen. Instead, report it once from
> > the boot CPU. Additionally removes the redundant "feature" word from the
> > "CPU features:" line.
> > 
> > Before (redundant "feature", and missing PAN emulation report):
> > 
> >  SMP: Total of 4 processors activated.
> >  CPU features: detected feature: 32-bit EL0 Support
> >  CPU features: detected feature: Kernel page table isolation (KPTI)
> >  CPU: All CPU(s) started at EL2
> > 
> > After:
> > 
> >  SMP: Total of 4 processors activated.
> >  CPU features: detected: 32-bit EL0 Support
> >  CPU features: detected: Kernel page table isolation (KPTI)
> >  CPU features: emulated: Privileged Access Never (PAN) using TTBR0_EL1 switching
> >  CPU: All CPU(s) started at EL2
> > 
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > ---
> >  arch/arm64/kernel/cpufeature.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index 29b1f873e337..6c799ca58b53 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -1333,9 +1333,6 @@ static void verify_local_cpu_capabilities(void)
> >  
> >  	if (system_supports_sve())
> >  		verify_sve_features();
> > -
> > -	if (system_uses_ttbr0_pan())
> > -		pr_info("Emulating Privileged Access Never (PAN) using TTBR0_EL1 switching\n");
> >  }
> >  
> >  void check_local_cpu_capabilities(void)
> > @@ -1360,7 +1357,7 @@ void check_local_cpu_capabilities(void)
> >  
> >  static void __init setup_feature_capabilities(void)
> >  {
> > -	update_cpu_capabilities(arm64_features, "detected feature:");
> > +	update_cpu_capabilities(arm64_features, "detected:");
> 
> Although I get what you're saying about redundant use of the word
> "features", this feels like cosmetic churn that is unrelated to the
> problem this patch is addressing.

Given it seems sensible, shall we just split that into a separate patch? 

FWIW, for a patch with just this change:

Acked-by: Mark Rutland <mark.rutland@....com>

> It could be worth reviewing the CPU errata messages and other
> miscellaneous printks together to make them less verbose and more
> consistent all in one go, but that would be a separate patch...
> 
> >  	enable_cpu_capabilities(arm64_features);
> >  }
> >  
> > @@ -1394,6 +1391,9 @@ void __init setup_cpu_features(void)
> >  	if (system_supports_32bit_el0())
> >  		setup_elf_hwcaps(compat_elf_hwcaps);
> >  
> > +	if (system_uses_ttbr0_pan())
> > +		pr_info("emulated: Privileged Access Never (PAN) using TTBR0_EL1 switching\n");
> > +
> 
> Moving this seems sensible.  The other option would be to paste it into
> update_cpu_capabilities(), but the message would still potentially get
> printed multiple times, so that doesn't feel like the right approach.

I think that more ideally, we'd give this an entry in the arm64_features array,
but because it's effectively a negative feature, it's a little tricky.

This also looks fine to me, so FWIW:

Acked-by: Mark Rutland <mark.rutland@....com>

Mark.