linux-kernel - Re: [PATCH v4 1/6] KVM: x86: Play nice with protected guests in complete_hypercall

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Message-ID: <e263259c-065e-43f6-8d5b-626e978cc6ab@amd.com>
Date: Fri, 29 Nov 2024 14:31:05 +0530
From: "Nikunj A. Dadhania" <nikunj@....com>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Tom Lendacky <thomas.lendacky@....com>, Binbin Wu
 <binbin.wu@...ux.intel.com>, Isaku Yamahata <isaku.yamahata@...el.com>,
 Kai Huang <kai.huang@...el.com>, Xiaoyao Li <xiaoyao.li@...el.com>
Subject: Re: [PATCH v4 1/6] KVM: x86: Play nice with protected guests in
 complete_hypercall_exit()

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@...r.kernel.org
> Cc: Tom Lendacky <thomas.lendacky@....com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Nikunj A Dadhania <nikunj@....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;