Subject: Re: [PATCH 1/1] KVM: inject data abort if instruction cannot be decoded

From: Marc Zyngier <[email-masked]>
Date: Thu, 05 Sep 2019 09:03:50 +0100

[Please use my kernel.org address. My arm.com address will disappear shortly]

On Wed, 04 Sep 2019 19:07:36 +0100,
Heinrich Schuchardt <[email-masked]> wrote:
> 
> If an application tries to access memory that is not mapped, an error
> ENOSYS, "load/store instruction decoding not implemented" may occur.
> QEMU will hang with a register dump.
> 
> Instead create a data abort that can be handled gracefully by the
> application running in the virtual environment.
> 
> Now the virtual machine can react to the event in the most appropriate
> way - by recovering, by writing an informative log, or by rebooting.
> 
> Signed-off-by: Heinrich Schuchardt <[email-masked]>
> ---
>  virt/kvm/arm/mmio.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/virt/kvm/arm/mmio.c b/virt/kvm/arm/mmio.c
> index a8a6a0c883f1..0cbed7d6a0f4 100644
> --- a/virt/kvm/arm/mmio.c
> +++ b/virt/kvm/arm/mmio.c
> @@ -161,8 +161,8 @@ 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;
> +		kvm_inject_dabt(vcpu, kvm_vcpu_get_hfar(vcpu));
> +		return 1;

How can you tell that the access would fault? You have no idea at that
stage (the kernel doesn't know about the MMIO ranges that userspace
handles). All you know is that you're faced with a memory access that
you cannot emulate in the kernel. Injecting a data abort at that stage
is not something that the architecture allows.

If you want to address this, consider forwarding the access to
userspace. You'll only need an instruction decoder (supporting T1, T2,
A32 and A64) and a S1 page table walker (one per page table format,
all three of them) to emulate the access (having taken care of
stopping all the other vcpus to make sure there is no concurrent
modification of the page tables). You'll then be able to return the
result of the access back to the kernel.

Of course, the best thing would be to actually fix the guest so that
it doesn't use non-emulatable MMIO accesses. In general, that the sign
of a bug in low-level accessors.

	M.

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

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