Re: [PATCH v3 for-8.2] i386/sev: Avoid SEV-ES crash due to missing MSR_EFER_LMA bit

[Paolo Bonzini <pbonzini@redhat.com>]

On 12/6/23 16:58, Michael Roth wrote:
> Commit 7191f24c7fcf ("accel/kvm/kvm-all: Handle register access errors")
> added error checking for KVM_SET_SREGS/KVM_SET_SREGS2. In doing so, it
> exposed a long-running bug in current KVM support for SEV-ES where the
> kernel assumes that MSR_EFER_LMA will be set explicitly by the guest
> kernel, in which case EFER write traps would result in KVM eventually
> seeing MSR_EFER_LMA get set and recording it in such a way that it would
> be subsequently visible when accessing it via KVM_GET_SREGS/etc.
> 
> However, guest kernels currently rely on MSR_EFER_LMA getting set
> automatically when MSR_EFER_LME is set and paging is enabled via
> CR0_PG_MASK. As a result, the EFER write traps don't actually expose the
> MSR_EFER_LMA bit, even though it is set internally, and when QEMU
> subsequently tries to pass this EFER value back to KVM via
> KVM_SET_SREGS* it will fail various sanity checks and return -EINVAL,
> which is now considered fatal due to the aforementioned QEMU commit.
> 
> This can be addressed by inferring the MSR_EFER_LMA bit being set when
> paging is enabled and MSR_EFER_LME is set, and synthesizing it to ensure
> the expected bits are all present in subsequent handling on the host
> side.
> 
> Ultimately, this handling will be implemented in the host kernel, but to
> avoid breaking QEMU's SEV-ES support when using older host kernels, the
> same handling can be done in QEMU just after fetching the register
> values via KVM_GET_SREGS*. Implement that here.
> 
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Marcelo Tosatti <mtosatti@redhat.com>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Cc: Akihiko Odaki <akihiko.odaki@daynix.com>
> Cc: Philippe Mathieu-Daudé <philmd@linaro.org>
> Cc: Lara Lazier <laramglazier@gmail.com>
> Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
> Cc: Maxim Levitsky <mlevitsk@redhat.com>
> Cc: kvm@vger.kernel.org
> Fixes: 7191f24c7fcf ("accel/kvm/kvm-all: Handle register access errors")
> Signed-off-by: Michael Roth <michael.roth@amd.com>
> ---
>   target/i386/kvm/kvm.c | 8 ++++++++
>   1 file changed, 8 insertions(+)
> 
> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
> index 11b8177eff..4ce80555b4 100644
> --- a/target/i386/kvm/kvm.c
> +++ b/target/i386/kvm/kvm.c
> @@ -3643,6 +3643,10 @@ static int kvm_get_sregs(X86CPU *cpu)
>       env->cr[4] = sregs.cr4;
>   
>       env->efer = sregs.efer;
> +    if (sev_es_enabled() && env->efer & MSR_EFER_LME &&
> +        env->cr[0] & CR0_PG_MASK) {
> +        env->efer |= MSR_EFER_LMA;
> +    }
>   
>       /* changes to apic base and cr8/tpr are read back via kvm_arch_post_run */
>       x86_update_hflags(env);
> @@ -3682,6 +3686,10 @@ static int kvm_get_sregs2(X86CPU *cpu)
>       env->cr[4] = sregs.cr4;
>   
>       env->efer = sregs.efer;
> +    if (sev_es_enabled() && env->efer & MSR_EFER_LME &&
> +        env->cr[0] & CR0_PG_MASK) {
> +        env->efer |= MSR_EFER_LMA;
> +    }
>   
>       env->pdptrs_valid = sregs.flags & KVM_SREGS2_FLAGS_PDPTRS_VALID;
>   

Acked-by: Paolo Bonzini <pbonzini@redhat.com>

Thanks.  We can change it for 9.0 (especially adding a comment, because 
long term having it only in the commit message isn't great) but for now 
it's safe.

Paolo