Re: [PATCH v4 1/6] KVM: x86: Play nice with protected guests in complete_hypercall_exit()

Nikunj A. Dadhania
2024-11-29T09:01:15Z

On 11/28/2024 6:13 AM, Sean Christopherson wrote:
> Use is_64_bit_hypercall() instead of is_64_bit_mode() to detect a 64-bit
> hypercall when completing said hypercall. For guests with protected state,
> e.g. SEV-ES and SEV-SNP, KVM must assume the hypercall was made in 64-bit
> mode as the vCPU state needed to detect 64-bit mode is unavailable.
>
> Hacking the sev_smoke_test selftest to generate a KVM_HC_MAP_GPA_RANGE
> hypercall via VMGEXIT trips the WARN:
>
> ------------[ cut here ]------------
> WARNING: CPU: 273 PID: 326626 at arch/x86/kvm/x86.h:180 complete_hypercall_exit+0x44/0xe0 [kvm]
> Modules linked in: kvm_amd kvm ... [last unloaded: kvm]
> CPU: 273 UID: 0 PID: 326626 Comm: sev_smoke_test Not tainted 6.12.0-smp--392e932fa0f3-feat #470
> Hardware name: Google Astoria/astoria, BIOS 0.20240617.0-0 06/17/2024
> RIP: 0010:complete_hypercall_exit+0x44/0xe0 [kvm]
> Call Trace:
> <TASK>
> kvm_arch_vcpu_ioctl_run+0x2400/0x2720 [kvm]
> kvm_vcpu_ioctl+0x54f/0x630 [kvm]
> __se_sys_ioctl+0x6b/0xc0
> do_syscall_64+0x83/0x160
> entry_SYSCALL_64_after_hwframe+0x76/0x7e
> </TASK>
> ---[ end trace 0000000000000000 ]---
>
> Fixes: b5aead0064f3 ("KVM: x86: Assume a 64-bit hypercall for guests with protected state")
> Cc: stable@vger.kernel.org
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Nikunj A Dadhania <nikunj@amd.com>

> ---
> arch/x86/kvm/x86.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 2e713480933a..0b2fe4aa04a2 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9976,7 +9976,7 @@ static int complete_hypercall_exit(struct kvm_vcpu *vcpu)
> {
> u64 ret = vcpu->run->hypercall.ret;
>
> - if (!is_64_bit_mode(vcpu))
> + if (!is_64_bit_hypercall(vcpu))
> ret = (u32)ret;
> kvm_rax_write(vcpu, ret);
> ++vcpu->stat.hypercalls;