Re: [PATCH v1 00/13] KVM: arm64: Fixed features for protected VMs

From: Andrew Jones <drjones@redhat.com>
To: Fuad Tabba <tabba@google.com>
Cc: kvmarm@lists.cs.columbia.edu, kernel-team@android.com,
	kvm@vger.kernel.org, maz@kernel.org, pbonzini@redhat.com,
	will@kernel.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v1 00/13] KVM: arm64: Fixed features for protected VMs
Date: Tue, 8 Jun 2021 17:07:39 +0200	[thread overview]
Message-ID: <20210608150739.7ztstw52ynxh6m5p@gator> (raw)
In-Reply-To: <20210608141141.997398-1-tabba@google.com>

On Tue, Jun 08, 2021 at 03:11:28PM +0100, Fuad Tabba wrote:
> Hi,
> 
> This patch series adds support for restricting CPU features for protected VMs
> in KVM [1].
> 
> Various feature configurations are allowed in KVM/arm64. Supporting all
> these features in pKVM is difficult, as it either involves moving much of
> the handling code to EL2, which adds bloat and results in a less verifiable
> trusted code base. Or it involves leaving the code handling at EL1, which
> risks having an untrusted host kernel feeding wrong information to the EL2
> and to the protected guests.
> 
> This series attempts to mitigate this by reducing the configuration space,
> providing a reduced amount of feature support at EL2 with the least amount of
> compromise of protected guests' capabilities.
> 
> This is done by restricting CPU features exposed to protected guests through
> feature registers. These restrictions are enforced by trapping register
> accesses as well as instructions associated with these features, and injecting
> an undefined exception into the guest if it attempts to use a restricted
> feature.
> 
> The features being restricted (only for protected VMs in protected mode) are
> the following:
> - Debug, Trace, and DoubleLock
> - Performance Monitoring (PMU)
> - Statistical Profiling (SPE)
> - Scalable Vector Extension (SVE)
> - Memory Partitioning and Monitoring (MPAM)
> - Activity Monitoring (AMU)
> - Memory Tagging (MTE)
> - Limited Ordering Regions (LOR)
> - AArch32 State
> - Generic Interrupt Controller (GIC) (depending on rVIC support)
> - Nested Virtualization (NV)
> - Reliability, Availability, and Serviceability (RAS) above V1
> - Implementation-defined Features

Hi Fuad,

I see this series takes the approach we currently have in KVM of masking
features we don't want to expose to the guest. This approach adds yet
another "reject list" to be maintained as hardware evolves. I'd rather see
that we first change KVM to using an accept list, i.e. mask everything and
then only set what we want to enable. Mimicking that new accept list in
pKVM, where much less would be enabled, would reduce the amount of
maintenance needed.

Thanks,
drew

> 
> This series is based on kvmarm/next and Will's patches for an Initial pKVM user
> ABI [1]. You can find the applied series here [2].
> 
> Cheers,
> /fuad
> 
> [1] https://lore.kernel.org/kvmarm/20210603183347.1695-1-will@kernel.org/
> 
> For more details about pKVM, please refer to Will's talk at KVM Forum 2020:
> https://www.youtube.com/watch?v=edqJSzsDRxk
> 
> [2] https://android-kvm.googlesource.com/linux/+/refs/heads/tabba/el2_fixed_feature_v1
> 
> To: kvmarm@lists.cs.columbia.edu
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Will Deacon <will@kernel.org>
> Cc: James Morse <james.morse@arm.com>
> Cc: Alexandru Elisei <alexandru.elisei@arm.com>
> Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Christoffer Dall <christoffer.dall@arm.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Quentin Perret <qperret@google.com>
> Cc: kvm@vger.kernel.org
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: kernel-team@android.com
> 
> Fuad Tabba (13):
>   KVM: arm64: Remove trailing whitespace in comments
>   KVM: arm64: MDCR_EL2 is a 64-bit register
>   KVM: arm64: Fix name of HCR_TACR to match the spec
>   KVM: arm64: Refactor sys_regs.h,c for nVHE reuse
>   KVM: arm64: Restore mdcr_el2 from vcpu
>   KVM: arm64: Add feature register flag definitions
>   KVM: arm64: Add config register bit definitions
>   KVM: arm64: Guest exit handlers for nVHE hyp
>   KVM: arm64: Add trap handlers for protected VMs
>   KVM: arm64: Move sanitized copies of CPU features
>   KVM: arm64: Trap access to pVM restricted features
>   KVM: arm64: Handle protected guests at 32 bits
>   KVM: arm64: Check vcpu features at pVM creation
> 
>  arch/arm64/include/asm/kvm_arm.h        |  34 +-
>  arch/arm64/include/asm/kvm_asm.h        |   2 +-
>  arch/arm64/include/asm/kvm_host.h       |   2 +-
>  arch/arm64/include/asm/kvm_hyp.h        |   4 +
>  arch/arm64/include/asm/sysreg.h         |   6 +
>  arch/arm64/kvm/arm.c                    |   4 +
>  arch/arm64/kvm/debug.c                  |   5 +-
>  arch/arm64/kvm/hyp/include/hyp/switch.h |  42 ++
>  arch/arm64/kvm/hyp/nvhe/Makefile        |   2 +-
>  arch/arm64/kvm/hyp/nvhe/debug-sr.c      |   2 +-
>  arch/arm64/kvm/hyp/nvhe/mem_protect.c   |   6 -
>  arch/arm64/kvm/hyp/nvhe/switch.c        | 114 +++++-
>  arch/arm64/kvm/hyp/nvhe/sys_regs.c      | 501 ++++++++++++++++++++++++
>  arch/arm64/kvm/hyp/vhe/debug-sr.c       |   2 +-
>  arch/arm64/kvm/pkvm.c                   |  31 ++
>  arch/arm64/kvm/sys_regs.c               |  62 +--
>  arch/arm64/kvm/sys_regs.h               |  35 ++
>  17 files changed, 782 insertions(+), 72 deletions(-)
>  create mode 100644 arch/arm64/kvm/hyp/nvhe/sys_regs.c
> 
> 
> base-commit: 35b256a5eebe3ac715b4ea6234aa4236a10d1a88
> -- 
> 2.32.0.rc1.229.g3e70b5a671-goog
> 
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