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From: Will Deacon <will.deacon@arm.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: Kristina Martsenko <kristina.martsenko@arm.com>,
	Mark Rutland <Mark.Rutland@arm.com>,
	"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
	Andrew Jones <drjones@redhat.com>,
	Jacob Bramley <Jacob.Bramley@arm.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Marc Zyngier <Marc.Zyngier@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Adam Wallis <awallis@codeaurora.org>,
	Suzuki Poulose <Suzuki.Poulose@arm.com>,
	Christoffer Dall <Christoffer.Dall@arm.com>,
	Dave P Martin <Dave.Martin@arm.com>,
	Amit Kachhap <Amit.Kachhap@arm.com>,
	Ramana Radhakrishnan <ramana.radhakrishnan@foss.arm.com>,
	"kvmarm@lists.cs.columbia.edu" <kvmarm@lists.cs.columbia.edu>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH v5 11/17] arm64: docs: document pointer authentication
Date: Fri, 19 Oct 2018 18:45:24 +0100
Message-ID: <20181019174524.GC4429@brain-police> (raw)
In-Reply-To: <20181019151029.GD3985@arrakis.emea.arm.com>

On Fri, Oct 19, 2018 at 04:10:29PM +0100, Catalin Marinas wrote:
> On Fri, Oct 19, 2018 at 03:42:23PM +0100, Kristina Martsenko wrote:
> > On 19/10/2018 12:35, Catalin Marinas wrote:
> > > On Tue, Oct 16, 2018 at 05:14:39PM +0100, Kristina Martsenko wrote:
> > >> On 05/10/2018 10:04, Ramana Radhakrishnan wrote:
> > >>> On 05/10/2018 09:47, Kristina Martsenko wrote:
> > >>>> +Virtualization
> > >>>> +--------------
> > >>>> +
> > >>>> +Pointer authentication is not currently supported in KVM guests. KVM
> > >>>> +will mask the feature bits from ID_AA64ISAR1_EL1, and attempted use of
> > >>>> +the feature will result in an UNDEFINED exception being injected into
> > >>>> +the guest.
> > >>>
> > >>> However applications using instructions from the hint space will
> > >>> continue to work albeit without any protection (as they would just be
> > >>> nops) ?
> > >>
> > >> Mostly, yes. If the guest leaves SCTLR_EL1.EnIA unset (and
> > >> EnIB/EnDA/EnDB), then PAC* and AUT* instructions in the HINT space will
> > >> execute as NOPs. If the guest sets EnIA, then PAC*/AUT* instructions
> > >> will trap and KVM will inject an "Unknown reason" exception into the
> > >> guest (which will cause a Linux guest to send a SIGILL to the application).
> > > 
> > > I think that part is fine. If KVM (a fairly recent version with CPUID
> > > sanitisation) does not enable ptr auth, the CPUID should not advertise
> > > this feature either so the guest kernel should not enable it. For the
> > > above instructions in the HINT space, they will just be NOPs. If the
> > > guest kernel enables the feature regardless of the CPUID information, it
> > > deserves to get an "Unknown reason" exception.
> > > 
> > >> In the latter case we could instead pretend the instruction was a NOP
> > >> and not inject an exception, but trapping twice per every function would
> > >> probably be terrible for performance. The guest shouldn't be setting
> > >> EnIA anyway if ID_AA64ISAR1_EL1 reports that pointer authentication is
> > >> not present (because KVM has hidden it).
> > > 
> > > I don't think we should. The SCTLR_EL1 bits are RES0 unless you know
> > > that the feature is present via CPUID.
> > > 
> > >> The other special case is the XPACLRI instruction, which is also in the
> > >> HINT space. Currently it will trap and KVM will inject an exception into
> > >> the guest. We should probably change this to NOP instead, as that's what
> > >> applications will expect. Unfortunately there is no EnIA-like control to
> > >> make it NOP.
> > > 
> > > Very good catch. Basically if EL2 doesn't know about ptr auth (older
> > > distro), EL1 may or may not know but leaves SCTLR_EL1 disabled (based on
> > > CPUID), the default HCR_EL2 is to trap (I'm ignoring EL3 as that's like
> > > to have ptr auth enabled, being built for the specific HW). So a user
> > > app considering XPACLRI a NOP (or inoffensive) will get a SIGILL
> > > (injected by the guest kernel following the injection of "Unknown
> > > reason" exception by KVM).
> > > 
> > > Ramana, is XPACLRI commonly generated by gcc and expects it to be a NOP?
> > > Could we restrict it to only being used at run-time if the corresponding
> > > HWCAP is set? This means redefining this instruction as no longer in the
> > > NOP space.
> > 
> > I think an alternative solution is to just disable trapping of pointer
> > auth instructions in KVM. This will mean that the instructions will
> > behave the same in the guest as they do in the host. HINT-space
> > instructions (including XPACLRI) will behave as NOPs (or perform their
> > function, if enabled by the guest), and will not trap.
> 
> OK, so this means disabling the trap (during early EL2 setup) but still
> sanitizing the CPUID not to report the feature to EL1 unless fully
> supported on all CPUs.

... which is perfectly sensible, but not actually my main concern here.
I'm worried about the possibility of distributions shipping *now* with
userspace that's built with these instructions. That stuff is going to
break if/when it encounters v8.3 hardware, and I don't think we can do
much about it other than alert them to the potential issue.

Will

  reply index

Thread overview: 62+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-05  8:47 [PATCH 00/17] ARMv8.3 pointer authentication support Kristina Martsenko
2018-10-05  8:47 ` [PATCH v5 01/17] arm64: add pointer authentication register bits Kristina Martsenko
2018-10-11 16:28   ` Will Deacon
2018-10-12  8:53     ` Mark Rutland
2018-10-12  8:56       ` Will Deacon
2018-10-12  9:50         ` Mark Rutland
2018-10-05  8:47 ` [PATCH v5 02/17] arm64/kvm: consistently handle host HCR_EL2 flags Kristina Martsenko
2018-10-05  8:47 ` [PATCH v5 03/17] arm64/kvm: hide ptrauth from guests Kristina Martsenko
2018-10-05  8:47 ` [PATCH v5 04/17] arm64: Don't trap host pointer auth use to EL2 Kristina Martsenko
2018-10-05  8:47 ` [PATCH v5 05/17] arm64/cpufeature: detect pointer authentication Kristina Martsenko
2018-10-05  8:47 ` [PATCH v5 06/17] asm-generic: mm_hooks: allow hooks to be overridden individually Kristina Martsenko
2018-10-05  8:47 ` [PATCH v5 07/17] arm64: add basic pointer authentication support Kristina Martsenko
2018-10-11 16:00   ` Suzuki K Poulose
2018-10-19 11:15   ` Catalin Marinas
2018-10-19 11:24     ` Will Deacon
2018-10-19 15:36       ` Kees Cook
2018-10-19 15:49         ` Will Deacon
2018-10-19 16:05           ` Kees Cook
2018-10-19 16:16             ` Will Deacon
2018-10-19 15:54         ` Mark Rutland
2018-10-19 16:49       ` Cyrill Gorcunov
2018-11-14 18:11       ` Will Deacon
2018-11-15 10:25         ` Dave Martin
2018-10-23  8:36     ` Ramana Radhakrishnan
2018-10-23 10:20       ` Will Deacon
2018-10-05  8:47 ` [PATCH v5 08/17] arm64: expose user PAC bit positions via ptrace Kristina Martsenko
2018-10-05  8:47 ` [PATCH v5 09/17] arm64: perf: strip PAC when unwinding userspace Kristina Martsenko
2018-10-05  8:47 ` [PATCH v5 10/17] arm64: enable pointer authentication Kristina Martsenko
2018-10-05  8:47 ` [PATCH v5 11/17] arm64: docs: document " Kristina Martsenko
2018-10-05  9:04   ` Ramana Radhakrishnan
2018-10-16 16:14     ` Kristina Martsenko
2018-10-19 11:35       ` Catalin Marinas
2018-10-19 11:47         ` Marc Zyngier
2018-10-19 12:22         ` Will Deacon
2018-10-19 14:42         ` Kristina Martsenko
2018-10-19 15:10           ` Catalin Marinas
2018-10-19 17:45             ` Will Deacon [this message]
2018-11-02  6:02               ` Jon Masters
2018-10-24 10:56         ` Ramana Radhakrishnan
2018-10-15 22:35   ` Kees Cook
2018-11-02  9:46     ` Ramana Radhakrishnan
2018-10-05  8:47 ` [RFC 12/17] arm64: move ptrauth keys to thread_info Kristina Martsenko
2018-10-19 11:38   ` Catalin Marinas
2018-10-05  8:47 ` [RFC 13/17] arm64: install user ptrauth keys at kernel exit time Kristina Martsenko
2018-10-05  8:47 ` [RFC 14/17] arm64: unwind: strip PAC from kernel addresses Kristina Martsenko
2018-10-05  8:47 ` [RFC 15/17] arm64: enable ptrauth earlier Kristina Martsenko
2018-10-06 12:51   ` Amit Kachhap
2018-10-05  8:47 ` [RFC 16/17] arm64: initialize and switch ptrauth kernel keys Kristina Martsenko
2018-10-06 12:56   ` Amit Kachhap
2018-10-05  8:47 ` [RFC 17/17] arm64: compile the kernel with ptrauth -msign-return-address Kristina Martsenko
2018-10-05  9:01   ` Ramana Radhakrishnan
2018-10-11 14:00     ` Kristina Martsenko
2018-10-11 14:23   ` Vladimir Murzin
2018-10-15 22:38     ` Kees Cook
2018-10-15 22:42 ` [PATCH 00/17] ARMv8.3 pointer authentication support Kees Cook
2018-11-13 16:17   ` Kristina Martsenko
2018-11-13 23:09     ` Kees Cook
2018-11-14 15:54       ` Kristina Martsenko
2018-11-14 21:47       ` Mark Rutland
2018-11-14 22:48         ` Kees Cook
2018-10-19 12:36 ` Will Deacon
2018-10-23  8:39   ` Ramana Radhakrishnan

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