From: Will Deacon <will@kernel.org>
To: Amit 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 1/2] arm64: ptrauth: add pointer authentication Armv8.6 enhanced feature
Date: Mon, 2 Mar 2020 16:29:48 +0000 [thread overview]
Message-ID: <20200302162948.GB11427@willie-the-truck> (raw)
In-Reply-To: <194f0008-4071-b143-f326-2f067e3b82fe@arm.com>
On Mon, Mar 02, 2020 at 06:18:17PM +0530, Amit Kachhap wrote:
> On 2/28/20 5:53 PM, Will Deacon wrote:
> > On Fri, Feb 28, 2020 at 12:03:14PM +0000, Mark Rutland wrote:
> > > On Fri, Feb 28, 2020 at 11:57:37AM +0000, Will Deacon wrote:
> > > > On Wed, Feb 19, 2020 at 06:30:39PM +0530, Amit Daniel Kachhap wrote:
> > > > > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > > > > index 8d1c979..a4f8adb 100644
> > > > > --- a/arch/arm64/kernel/cpufeature.c
> > > > > +++ b/arch/arm64/kernel/cpufeature.c
> > > > > @@ -154,9 +154,9 @@ static const struct arm64_ftr_bits ftr_id_aa64isar1[] = {
> > > > > ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64ISAR1_FCMA_SHIFT, 4, 0),
> > > > > ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64ISAR1_JSCVT_SHIFT, 4, 0),
> > > > > ARM64_FTR_BITS(FTR_VISIBLE_IF_IS_ENABLED(CONFIG_ARM64_PTR_AUTH),
> > > > > - FTR_STRICT, FTR_LOWER_SAFE, ID_AA64ISAR1_API_SHIFT, 4, 0),
> > > > > + FTR_STRICT, FTR_EXACT, ID_AA64ISAR1_API_SHIFT, 4, 0),
> > > > > ARM64_FTR_BITS(FTR_VISIBLE_IF_IS_ENABLED(CONFIG_ARM64_PTR_AUTH),
> > > > > - FTR_STRICT, FTR_LOWER_SAFE, ID_AA64ISAR1_APA_SHIFT, 4, 0),
> > > > > + FTR_STRICT, FTR_EXACT, ID_AA64ISAR1_APA_SHIFT, 4, 0),
> > > > > ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64ISAR1_DPB_SHIFT, 4, 0),
> > > > > ARM64_FTR_END,
> > > >
> > > > Hmm. This is a user-visible change and should probably be in its own patch.
> > > > It also means we will no longer advertise PAC on systems where not all of
> > > > the cores have "Enhanced PAC"; is that really necessary?
> > >
> > > It matters for KVM, since a guest won't expect the enhanced PAC trap if
> > > the ID registers say it does not have it.
> > >
> > > For userspace, the difference is it'll get a SIGILL on the AUT*
> > > instruction rather than a SIGSEGV when using the result of the AUT*
> > > instruction.
> >
> > Yes, if PAC is enabled.
> >
> > > > Generally we rely on incremental updates to unsigned ID register fields
> > > > being a superset (i.e. compatible with) the old behaviour. If that's not
> > > > the case here, then older kernels are broken and we may need new HWCAPs.
> > >
> > > In this case, the behaviour isn't a strict superset. Enhanced PAC
> > > unconditionally changed the behaviour of AUT* (i.e. there's no opt-in
> > > with a control bit), but it's not clear to me how much this matters for
> > > userspace.
> >
> > Doesn't that violate D13.1.3 "Principles of the ID scheme for fields in
> > ID registers"?
> >
> > The part I dislike is that older kernels will happily advertise PAC to
> > userspace on a system with mismatched legacy/enhanced PAC, and so the
> > change above doesn't make sense for userspace because the HWCAPs are
> > already unreliable.
>
> How to got about it? Shall I send this part as a separate fix patch
> mentioning the requirement for doing it?
I didn't see a reply from Mark, but yes, I think this should be a separate
patch. Further, I think it should only affect KVM and not userspace.
Will
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next prev parent reply other threads:[~2020-03-02 16:29 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-19 13:00 [PATCH 0/2] arm64: add Armv8.6 pointer authentication Amit Daniel Kachhap
2020-02-19 13:00 ` [PATCH 1/2] arm64: ptrauth: add pointer authentication Armv8.6 enhanced feature Amit Daniel Kachhap
2020-02-28 11:57 ` Will Deacon
2020-02-28 12:03 ` Mark Rutland
2020-02-28 12:23 ` Will Deacon
2020-03-02 12:48 ` Amit Kachhap
2020-03-02 16:29 ` Will Deacon [this message]
2020-02-19 13:00 ` [PATCH 2/2] arm64: kprobe: disable probe of fault prone ptrauth instruction Amit Daniel Kachhap
2020-02-27 16:48 ` Mark Rutland
2020-02-28 11:12 ` Amit Kachhap
2020-02-28 11:18 ` Will Deacon
2020-02-28 11:31 ` Mark Rutland
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