Re: [PATCH 2/2] arm64: kprobe: disable probe of fault prone ptrauth instruction

From: Mark Rutland <mark.rutland@arm.com>
To: Will Deacon <will@kernel.org>
Cc: Kees Cook <keescook@chromium.org>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Kristina Martsenko <kristina.martsenko@arm.com>,
	Mark Brown <broonie@kernel.org>,
	James Morse <james.morse@arm.com>,
	Amit Kachhap <amit.kachhap@arm.com>,
	Vincenzo Frascino <Vincenzo.Frascino@arm.com>,
	Dave Martin <Dave.Martin@arm.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 2/2] arm64: kprobe: disable probe of fault prone ptrauth instruction
Date: Fri, 28 Feb 2020 11:31:01 +0000	[thread overview]
Message-ID: <20200228113100.GC36089@lakrids.cambridge.arm.com> (raw)
In-Reply-To: <20200228111856.GB2941@willie-the-truck>

On Fri, Feb 28, 2020 at 11:18:59AM +0000, Will Deacon wrote:
> On Fri, Feb 28, 2020 at 04:42:10PM +0530, Amit Kachhap wrote:
> > On 2/27/20 10:18 PM, Mark Rutland wrote:
> > > On Wed, Feb 19, 2020 at 06:30:40PM +0530, Amit Daniel Kachhap wrote:
> > > > diff --git a/arch/arm64/kernel/insn.c b/arch/arm64/kernel/insn.c
> > > > index 4a9e773..87f7c8a 100644
> > > > --- a/arch/arm64/kernel/insn.c
> > > > +++ b/arch/arm64/kernel/insn.c
> > > > @@ -63,6 +63,7 @@ bool __kprobes aarch64_insn_is_nop(u32 insn)
> > > >   	case AARCH64_INSN_HINT_WFI:
> > > >   	case AARCH64_INSN_HINT_SEV:
> > > >   	case AARCH64_INSN_HINT_SEVL:
> > > > +	case AARCH64_INSN_HINT_AUTIASP:
> > > >   		return false;
> > > >   	default:
> > > >   		return true;
> > > 
> > > I'm afraid that the existing code here is simply wrong, and this is
> > > adding to the mess.
> > > 
> > > We have no idea what HINT space instructions will be in the future, so
> > > the only sensible implementations of aarch64_insn_is_nop() are something
> > > like:
> > > 
> > > bool __kprobes aarch64_insn_is_nop(u32 insn)
> > > {
> > > 	return insn == aarch64_insn_gen_hint(AARCH64_INSN_HINT_NOP);
> > > }
> > > 
> > > ... and if we want to check for other HINT instructions, they should be
> > > checked explicitly.
> > > 
> > > Can you please change aarch64_insn_is_nop() as above?
> > 
> > Agree that current implementation is unclear.
> > I will implement as you suggested.
> 
> Well, please don't throw the baby out with the bath water. The existing
> code might be badly structured, but I think it's going a bit far to say
> it's "simply wrong". We need a whitelist /somewhere/ and I'd prefer not
> to regress the existing behaviour just because the whitelist is embedded
> in a function with "is_nop" in the name.

Ok; we can follow up with a more complete cleanup after these patches
have been merged.

Thanks,
Mark.

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