Re: [PATCH 1/2] KVM: arm/arm64: Allow reporting non-ISV data aborts to userspace

[Marc Zyngier <maz@kernel.org>]

On 09/09/2019 13:13, Christoffer Dall wrote:
> For a long time, if a guest accessed memory outside of a memslot using
> any of the load/store instructions in the architecture which doesn't
> supply decoding information in the ESR_EL2 (the ISV bit is not set), the
> kernel would print the following message and terminate the VM as a
> result of returning -ENOSYS to userspace:
> 
>   load/store instruction decoding not implemented
> 
> The reason behind this message is that KVM assumes that all accesses
> outside a memslot is an MMIO access which should be handled by
> userspace, and we originally expected to eventually implement some sort
> of decoding of load/store instructions where the ISV bit was not set.
> 
> However, it turns out that many of the instructions which don't provide
> decoding information on abort are not safe to use for MMIO accesses, and
> the remaining few that would potentially make sense to use on MMIO
> accesses, such as those with register writeback, are not used in
> practice.  It also turns out that fetching an instruction from guest
> memory can be a pretty horrible affair, involving stopping all CPUs on
> SMP systems, handling multiple corner cases of address translation in
> software, and more.  It doesn't appear likely that we'll ever implement
> this in the kernel.
> 
> What is much more common is that a user has misconfigured his/her guest
> and is actually not accessing an MMIO region, but just hitting some
> random hole in the IPA space.  In this scenario, the error message above
> is almost misleading and has led to a great deal of confusion over the
> years.
> 
> It is, nevertheless, ABI to userspace, and we therefore need to
> introduce a new capability that userspace explicitly enables to change
> behavior.
> 
> This patch introduces KVM_CAP_ARM_NISV_TO_USER (NISV meaning Non-ISV)
> which does exactly that, and introduces a new exit reason to report the
> event to userspace.  User space can then emulate an exception to the
> guest, restart the guest, suspend the guest, or take any other
> appropriate action as per the policy of the running system.
> 
> Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
> Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>
> ---
>  Documentation/virt/kvm/api.txt       | 29 ++++++++++++++++++++++++++++
>  arch/arm/include/asm/kvm_arm.h       |  2 ++
>  arch/arm/include/asm/kvm_emulate.h   |  5 +++++
>  arch/arm/include/asm/kvm_host.h      |  8 ++++++++
>  arch/arm64/include/asm/kvm_emulate.h |  5 +++++
>  arch/arm64/include/asm/kvm_host.h    |  8 ++++++++
>  include/uapi/linux/kvm.h             |  7 +++++++
>  virt/kvm/arm/arm.c                   | 21 ++++++++++++++++++++
>  virt/kvm/arm/mmio.c                  | 11 +++++++++--
>  9 files changed, 94 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/virt/kvm/api.txt b/Documentation/virt/kvm/api.txt
> index 2d067767b617..02501333f746 100644
> --- a/Documentation/virt/kvm/api.txt
> +++ b/Documentation/virt/kvm/api.txt
> @@ -4453,6 +4453,35 @@ Hyper-V SynIC state change. Notification is used to remap SynIC
>  event/message pages and to enable/disable SynIC messages/events processing
>  in userspace.
>  
> +		/* KVM_EXIT_ARM_NISV */
> +		struct {
> +			__u64 esr_iss;
> +			__u64 fault_ipa;
> +		} arm_nisv;
> +
> +Used on arm and arm64 systems. If a guest accesses memory not in a memslot,
> +KVM will typically return to userspace and ask it to do MMIO emulation on its
> +behalf. However, for certain classes of instructions, no instruction decode
> +(direction, length of memory access) is provided, and fetching and decoding
> +the instruction from the VM is overly complicated to live in the kernel.
> +
> +Historically, when this situation occurred, KVM would print a warning and kill
> +the VM. KVM assumed that if the guest accessed non-memslot memory, it was
> +trying to do I/O, which just couldn't be emulated, and the warning message was
> +phrased accordingly. However, what happened more often was that a guest bug
> +caused access outside the guest memory areas which should lead to a more
> +mearningful warning message and an external abort in the guest, if the access

meaningful?

> +did not fall within an I/O window.
> +
> +Userspace implementations can query for KVM_CAP_ARM_NISV_TO_USER, and enable
> +this capability at VM creation. Once this is done, these types of errors will
> +instead return to userspace with KVM_EXIT_ARM_NISV, with the valid bits from
> +the HSR (arm) and ESR_EL2 (arm64) in the esr_iss field, and the faulting IPA
> +in the fault_ipa field. Userspace can either fix up the access if it's
> +actually an I/O access by decoding the instruction from guest memory (if it's
> +very brave) and continue executing the guest, or it can decide to suspend,
> +dump, or restart the guest.
> +
>  		/* Fix the size of the union. */
>  		char padding[256];
>  	};
> diff --git a/arch/arm/include/asm/kvm_arm.h b/arch/arm/include/asm/kvm_arm.h
> index 0125aa059d5b..ce61b3b0058d 100644
> --- a/arch/arm/include/asm/kvm_arm.h
> +++ b/arch/arm/include/asm/kvm_arm.h
> @@ -162,6 +162,8 @@
>  #define HSR_ISV		(_AC(1, UL) << HSR_ISV_SHIFT)
>  #define HSR_SRT_SHIFT	(16)
>  #define HSR_SRT_MASK	(0xf << HSR_SRT_SHIFT)
> +#define HSR_CM		(1 << 8)
> +#define HSR_WNR		(1 << 6)

I think we already have that one (bizarrely placed after HSR_SSE).

>  #define HSR_FSC		(0x3f)
>  #define HSR_FSC_TYPE	(0x3c)
>  #define HSR_SSE		(1 << 21)
> diff --git a/arch/arm/include/asm/kvm_emulate.h b/arch/arm/include/asm/kvm_emulate.h
> index 40002416efec..e8ef349c04b4 100644
> --- a/arch/arm/include/asm/kvm_emulate.h
> +++ b/arch/arm/include/asm/kvm_emulate.h
> @@ -167,6 +167,11 @@ static inline bool kvm_vcpu_dabt_isvalid(struct kvm_vcpu *vcpu)
>  	return kvm_vcpu_get_hsr(vcpu) & HSR_ISV;
>  }
>  
> +static inline unsigned long kvm_vcpu_dabt_iss_nisv_sanitized(const struct kvm_vcpu *vcpu)
> +{
> +	return kvm_vcpu_get_hsr(vcpu) & (HSR_CM | HSR_WNR | HSR_FSC);
> +}
> +
>  static inline bool kvm_vcpu_dabt_iswrite(struct kvm_vcpu *vcpu)
>  {
>  	return kvm_vcpu_get_hsr(vcpu) & HSR_WNR;
> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> index 8a37c8e89777..19a92c49039c 100644
> --- a/arch/arm/include/asm/kvm_host.h
> +++ b/arch/arm/include/asm/kvm_host.h
> @@ -76,6 +76,14 @@ struct kvm_arch {
>  
>  	/* Mandated version of PSCI */
>  	u32 psci_version;
> +
> +	/*
> +	 * If we encounter a data abort without valid instruction syndrome
> +	 * information, report this to user space.  User space can (and
> +	 * should) opt in to this feature if KVM_CAP_ARM_NISV_TO_USER is
> +	 * supported.
> +	 */
> +	bool return_nisv_io_abort_to_user;
>  };
>  
>  #define KVM_NR_MEM_OBJS     40
> diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
> index d69c1efc63e7..a3c967988e1d 100644
> --- a/arch/arm64/include/asm/kvm_emulate.h
> +++ b/arch/arm64/include/asm/kvm_emulate.h
> @@ -258,6 +258,11 @@ static inline bool kvm_vcpu_dabt_isvalid(const struct kvm_vcpu *vcpu)
>  	return !!(kvm_vcpu_get_hsr(vcpu) & ESR_ELx_ISV);
>  }
>  
> +static inline unsigned long kvm_vcpu_dabt_iss_nisv_sanitized(const struct kvm_vcpu *vcpu)
> +{
> +	return kvm_vcpu_get_hsr(vcpu) & (ESR_ELx_CM | ESR_ELx_WNR | ESR_ELx_FSC);
> +}
> +
>  static inline bool kvm_vcpu_dabt_issext(const struct kvm_vcpu *vcpu)
>  {
>  	return !!(kvm_vcpu_get_hsr(vcpu) & ESR_ELx_SSE);
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index f656169db8c3..019bc560edc1 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -83,6 +83,14 @@ struct kvm_arch {
>  
>  	/* Mandated version of PSCI */
>  	u32 psci_version;
> +
> +	/*
> +	 * If we encounter a data abort without valid instruction syndrome
> +	 * information, report this to user space.  User space can (and
> +	 * should) opt in to this feature if KVM_CAP_ARM_NISV_TO_USER is
> +	 * supported.
> +	 */
> +	bool return_nisv_io_abort_to_user;
>  };
>  
>  #define KVM_NR_MEM_OBJS     40
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 5e3f12d5359e..dd79235b6435 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -235,6 +235,7 @@ struct kvm_hyperv_exit {
>  #define KVM_EXIT_S390_STSI        25
>  #define KVM_EXIT_IOAPIC_EOI       26
>  #define KVM_EXIT_HYPERV           27
> +#define KVM_EXIT_ARM_NISV         28
>  
>  /* For KVM_EXIT_INTERNAL_ERROR */
>  /* Emulate instruction failed. */
> @@ -392,6 +393,11 @@ struct kvm_run {
>  		} eoi;
>  		/* KVM_EXIT_HYPERV */
>  		struct kvm_hyperv_exit hyperv;
> +		/* KVM_EXIT_ARM_NISV */
> +		struct {
> +			__u64 esr_iss;
> +			__u64 fault_ipa;
> +		} arm_nisv;
>  		/* Fix the size of the union. */
>  		char padding[256];
>  	};
> @@ -996,6 +1002,7 @@ struct kvm_ppc_resize_hpt {
>  #define KVM_CAP_ARM_PTRAUTH_ADDRESS 171
>  #define KVM_CAP_ARM_PTRAUTH_GENERIC 172
>  #define KVM_CAP_PMU_EVENT_FILTER 173
> +#define KVM_CAP_ARM_NISV_TO_USER 174
>  
>  #ifdef KVM_CAP_IRQ_ROUTING
>  
> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> index 35a069815baf..7153504bb106 100644
> --- a/virt/kvm/arm/arm.c
> +++ b/virt/kvm/arm/arm.c
> @@ -98,6 +98,26 @@ int kvm_arch_check_processor_compat(void)
>  	return 0;
>  }
>  
> +int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
> +			    struct kvm_enable_cap *cap)
> +{
> +	int r;
> +
> +	if (cap->flags)
> +		return -EINVAL;
> +
> +	switch (cap->cap) {
> +	case KVM_CAP_ARM_NISV_TO_USER:
> +		r = 0;
> +		kvm->arch.return_nisv_io_abort_to_user = true;
> +		break;
> +	default:
> +		r = -EINVAL;
> +		break;
> +	}
> +
> +	return r;
> +}
>  
>  /**
>   * kvm_arch_init_vm - initializes a VM data structure
> @@ -196,6 +216,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  	case KVM_CAP_MP_STATE:
>  	case KVM_CAP_IMMEDIATE_EXIT:
>  	case KVM_CAP_VCPU_EVENTS:
> +	case KVM_CAP_ARM_NISV_TO_USER:
>  		r = 1;
>  		break;
>  	case KVM_CAP_ARM_SET_DEVICE_ADDR:
> diff --git a/virt/kvm/arm/mmio.c b/virt/kvm/arm/mmio.c
> index 6af5c91337f2..7b92e2744fa7 100644
> --- a/virt/kvm/arm/mmio.c
> +++ b/virt/kvm/arm/mmio.c
> @@ -167,8 +167,15 @@ int io_mem_abort(struct kvm_vcpu *vcpu, struct kvm_run *run,
>  		if (ret)
>  			return ret;
>  	} else {
> -		kvm_err("load/store instruction decoding not implemented\n");
> -		return -ENOSYS;
> +		if (vcpu->kvm->arch.return_nisv_io_abort_to_user) {
> +			run->exit_reason = KVM_EXIT_ARM_NISV;
> +			run->arm_nisv.esr_iss = kvm_vcpu_dabt_iss_nisv_sanitized(vcpu);
> +			run->arm_nisv.fault_ipa = fault_ipa;
> +			return 0;
> +		} else {
> +			kvm_info("Encountered data abort outside memslots with no valid syndrome info\n");

Nit: you could lose the else clause, making this line a bit shorter.

More importantly, I'd like this to become a rate-limited output, just to
keep things sane. How about kvm_pr_unimpl()?

> +			return -ENOSYS;
> +		}
>  	}
>  
>  	rt = vcpu->arch.mmio_decode.rt;
> 

Other than this collection of nits, this looks pretty good, and I'd
really like to take this patch in 5.5.

Thanks,

	M.
-- 
Jazz is not dead, it just smells funny...
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