Re: [PATCH] i386/kvm: Set return value after handling KVM_EXIT_HYPERCALL

[Xiaoyao Li]

On 12/12/2024 11:26 AM, Binbin Wu wrote:
> Userspace should set the ret field of hypercall after handling
> KVM_EXIT_HYPERCALL.  Otherwise, a stale value could be returned to KVM.
>
> Fixes: 47e76d03b15 ("i386/kvm: Add KVM_EXIT_HYPERCALL handling for 
> KVM_HC_MAP_GPA_RANGE")
> Reported-by: Farrah Chen <farrah.chen@intel.com>
> Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
> Tested-by: Farrah Chen <farrah.chen@intel.com>
> ---
> To test the TDX code in kvm-coco-queue, please apply the patch to the QEMU,
> otherwise, TDX guest boot could fail.
> A matching QEMU tree including this patch is here:
> https://github.com/intel-staging/qemu-tdx/releases/tag/tdx-qemu-upstream-v6.1-fix_kvm_hypercall_return_value
>
> Previously, the issue was not triggered because no one would modify the ret
> value. But with the refactor patch for __kvm_emulate_hypercall() in KVM,
> https://lore.kernel.org/kvm/20241128004344.4072099-7-seanjc@google.com/, the
> value could be modified.
> ---
>   target/i386/kvm/kvm.c | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
> index 8e17942c3b..4bcccb48d1 100644
> --- a/target/i386/kvm/kvm.c
> +++ b/target/i386/kvm/kvm.c
> @@ -6005,10 +6005,14 @@ static int kvm_handle_hc_map_gpa_range(struct kvm_run 
> *run)
>   
>   static int kvm_handle_hypercall(struct kvm_run *run)
>   {
> +    int ret = -EINVAL;
> +
>       if (run->hypercall.nr == KVM_HC_MAP_GPA_RANGE)
> -        return kvm_handle_hc_map_gpa_range(run);
> +        ret = kvm_handle_hc_map_gpa_range(run);
> +
> +    run->hypercall.ret = ret;

Updating run->hypercall.ret is useful only when QEMU needs to re-enter 
the guest. For the case of ret < 0, QEMU will stop the vcpu.

I think we might need re-think on the handling of KVM_EXIT_HYPERCALL. 
E.g., in what error case should QEMU stop the vcpu, and in what case can 
QEMU return the error back to the guest via run->hypercall.ret.

> -    return -EINVAL;
> +    return ret;
>   }
>   
>   #define VMX_INVALID_GUEST_STATE 0x80000021
>
> base-commit: ae35f033b874c627d81d51070187fbf55f0bf1a7