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

[Will Deacon]

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