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

[Christoffer Dall <christoffer.dall@arm.com>]

On Thu, Sep 26, 2019 at 02:47:55PM +0100, Marc Zyngier wrote:
> 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).
> 

Hey, at least the bit definitions were the same.

> >  #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()?
> 

Yes, I also got rid of 'Encountered' which doesn't seem to add much.

> > +			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,

    Christoffer
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