Re: [PATCH v3 2/3] arm64: cpufeature: Modify address authentication cpufeature to exact

From: Will Deacon <will@kernel.org>
To: Amit Daniel Kachhap <amit.kachhap@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	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>,
	Vincenzo Frascino <Vincenzo.Frascino@arm.com>,
	Dave Martin <Dave.Martin@arm.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v3 2/3] arm64: cpufeature: Modify address authentication cpufeature to exact
Date: Wed, 24 Jun 2020 08:49:23 +0100	[thread overview]
Message-ID: <20200624074923.GA5853@willie-the-truck> (raw)
In-Reply-To: <b2268521-9829-8aa5-feca-32a1f6f9e0a5@arm.com>

On Wed, Jun 24, 2020 at 12:43:20PM +0530, Amit Daniel Kachhap wrote:
> On 6/23/20 8:17 PM, Dave Martin wrote:
> > On Tue, Jun 23, 2020 at 06:47:02PM +0530, Amit Daniel Kachhap wrote:
> > > On 6/22/20 8:05 PM, Dave Martin wrote:
> > > > On Thu, Jun 18, 2020 at 10:40:28AM +0530, Amit Daniel Kachhap wrote:
> > > > > This patch modifies the address authentication cpufeature type to EXACT
> > > > >from earlier LOWER_SAFE as the different configurations added for Armv8.6
> > > > > enhanced PAC have different behaviour and there is no tunable to enable the
> > > > > lower safe versions.

$ git grep 'This patch' Documentation/process/submitting-patches.rst

> > > > Does a corresponding patch need to go to stable?  As things stand, I
> > > > think older kernels that support pointer auth will go wrong on v8.7+
> > > > hardware that has these features.
> 
> Yes It makes to add this patch to the stable version.
> 
> @Catalin, @Will - Shall I send this one with a fix subject line? Please let
> me know your suggestion.

What exactly goes wrong? As far as I can tell, we will taint and probably
(?) crash shortly afterwards if you run an old kernel on hardware with
mismatched pointer auth. If that's the case, I don't think this warrants
stable (especially since this file has seen enough churn that the backports
are likely risky).

Can you confirm that things only go wrong if we have a mismatched system,
and ideally provide an example of what the crash looks like (you should
be able to get a fast model to emulate this setup)?

Cheers,

Will

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