Re: [PATCH kvmtool v2 1/2] cpu: vmexit: Handle KVM_EXIT_UNKNOWN exit reason correctly

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi Aneesh,

On Mon, Feb 24, 2025 at 02:39:59PM +0530, Aneesh Kumar K.V (Arm) wrote:
> The return value for kernel VM exit handlers is confusing and has led to
> errors in different kernel exit handlers. A return value of 0 indicates
> a return to the VMM, whereas a return value of 1 indicates resuming
> execution in the guest. Some handlers mistakenly return 0 to force a
> return to the guest.
> 
> This worked in kvmtool because the exit_reason defaulted to
> 0 (KVM_EXIT_UNKNOWN), and kvmtool did not error out on an unknown exit
> reason. However, forcing a VMM exit with error on KVM_EXIT_UNKNOWN
> exit_reson would help catch these bugs early.

I think I understand what you're saying - if there's a bug in handle_exit()
in KVM that triggers an erroneous exit to userspace, exit_reason and the
exit information struct could still be at their default values, which are 0
from when kvm_run was allocated (in kvm_vm_ioctl_create_vcpu()).

An exit_reason of 0 is interpreted by userspace as KVM_EXIT_UNKNOWN, but
kvmtool on KVM_EXIT_UNKNOWN resumes the guest instead of signalling the
error, thus masking the buggy KVM behaviour.

The patch looks good to me:

Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>

Thanks,
Alex

> 
> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
> ---
>  kvm-cpu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kvm-cpu.c b/kvm-cpu.c
> index f66dcd07220c..7c62bfc56679 100644
> --- a/kvm-cpu.c
> +++ b/kvm-cpu.c
> @@ -170,7 +170,7 @@ int kvm_cpu__start(struct kvm_cpu *cpu)
>  
>  		switch (cpu->kvm_run->exit_reason) {
>  		case KVM_EXIT_UNKNOWN:
> -			break;
> +			goto panic_kvm;
>  		case KVM_EXIT_DEBUG:
>  			kvm_cpu__show_registers(cpu);
>  			kvm_cpu__show_code(cpu);
> -- 
> 2.43.0
>