Re: [PATCH 4/4] igvm/sev: forward the IGVM guest policy to the platform before launch

[Luigi Leonardi <leonardi@redhat.com>]

Hi Ani,

On Thu, Sep 03, 2026 at 02:16:30PM +0530, Ani Sinha wrote:
>
>
>> On 1 Sep 2026, at 3:39 PM, Luigi Leonardi <leonardi@redhat.com> wrote:
>>
>> The guest policy carried in the IGVM guest-policy initialization header
>> was parsed into QIgvm but never forwarded to the confidential guest
>> platform: the previous callback ran at the end of qigvm_process_file,
>> after LAUNCH_START had already been issued, so writing the policy had
>> no effect.
>>
>> Add a set_guest_policy callback and invoke it from the guest-policy
>> initialization handler, so the policy reaches the platform before
>> LAUNCH_START.
>>
>> The guest policy can also be set on the command line. As it is part of
>> the attestation report, silently overriding it would cause attestation
>> to fail, so return an error if the command-line value differs from the
>> one supplied by the IGVM file.
>>
>> Link: https://gitlab.com/qemu-project/qemu/-/work_items/4189
>> Fixes: 915b47078d ("backends/igvm: Handle policy for SEV guests")
>> Signed-off-by: Luigi Leonardi <leonardi@redhat.com>
>> ---
>> backends/confidential-guest-support.c |  9 ++++++++
>> backends/igvm.c                       |  4 ++++
>> target/i386/sev.c                     | 39 +++++++++++++++++++++++++++++++++++
>> 3 files changed, 52 insertions(+)
>>
>> diff --git a/backends/confidential-guest-support.c b/backends/confidential-guest-support.c
>> index a0b36d2da5..c0d15b4a76 100644
>> --- a/backends/confidential-guest-support.c
>> +++ b/backends/confidential-guest-support.c
>> @@ -38,6 +38,14 @@ static int set_guest_state(hwaddr gpa, uint8_t *ptr, uint64_t len,
>>     return -1;
>> }
>>
>> +static int set_guest_policy(ConfidentialGuestPolicyType policy_type,
>> +                            uint64_t policy, Error **errp)
>> +{
>> +    error_setg(errp,
>> +               "Setting guest policy is not supported for this platform");
>> +    return -1;
>> +}
>> +
>> static int set_id_block(void *id_block, uint32_t id_block_size,
>>                         void *id_auth, uint32_t id_auth_size,
>>                         Error **errp)
>> @@ -62,6 +70,7 @@ static void confidential_guest_support_class_init(ObjectClass *oc,
>>     ConfidentialGuestSupportClass *cgsc = CONFIDENTIAL_GUEST_SUPPORT_CLASS(oc);
>>     cgsc->check_support = check_support;
>>     cgsc->set_guest_state = set_guest_state;
>> +    cgsc->set_guest_policy = set_guest_policy;
>>     cgsc->set_id_block = set_id_block;
>>     cgsc->get_mem_map_entry = get_mem_map_entry;
>> }
>> diff --git a/backends/igvm.c b/backends/igvm.c
>> index 6545382546..5131ee7829 100644
>> --- a/backends/igvm.c
>> +++ b/backends/igvm.c
>> @@ -868,6 +868,10 @@ static int qigvm_initialization_guest_policy(QIgvm *ctx,
>>
>>     if (guest->compatibility_mask & ctx->compatibility_mask) {
>>         ctx->sev_policy = guest->policy;
>> +        if (ctx->cgsc) {
>> +            return ctx->cgsc->set_guest_policy(GUEST_POLICY_SEV,
>> +                                               guest->policy, errp);
>> +        }
>>     }
>>     return 0;
>> }
>> diff --git a/target/i386/sev.c b/target/i386/sev.c
>> index c76cdba8d2..533ea4b54e 100644
>> --- a/target/i386/sev.c
>> +++ b/target/i386/sev.c
>> @@ -128,6 +128,8 @@ struct SevCommonState {
>>     bool kernel_hashes;
>>     uint64_t sev_features;
>>     uint64_t supported_sev_features;
>> +    /* whether the guest policy was explicitly set on the command line */
>> +    bool policy_set;
>>
>>     /* runtime state */
>>     uint8_t api_major;
>> @@ -2723,6 +2725,40 @@ static int cgs_get_mem_map_entry(int index,
>>     return 0;
>> }
>>
>> +static int cgs_set_guest_policy(ConfidentialGuestPolicyType policy_type,
>> +                                uint64_t policy, Error **errp)
>> +{
>> +    SevCommonState *sev_common = SEV_COMMON(MACHINE(qdev_get_machine())->cgs);
>> +
>> +    if (policy_type != GUEST_POLICY_SEV) {
>> +        error_setg(errp, "SEV: Invalid guest policy type provided for SEV: %d",
>> +                   policy_type);
>> +        return -1;
>> +    }
>> +
>> +    if (sev_snp_enabled()) {
>> +        SevSnpGuestState *sev_snp_guest = SEV_SNP_GUEST(sev_common);
>> +
>> +        if (sev_common->policy_set &&
>> +            sev_snp_guest->kvm_start_conf.policy != policy) {
>> +            error_setg(errp, "SNP: policy mismatch between IGVM and CLI");
>> +            return -1;
>> +        }
>> +
>> +        sev_snp_guest->kvm_start_conf.policy = policy;

Thanks for you review!

>
>I had fixed a bug initially here where we need to check if the policy passed is not 0. I am not sure if that fix is still needed here.
>Please test this scenario:
>a) Generate an IGVM file with policy set to 0.

0 is not a valid sev-snp guest policy: bit 17 is reserved and must be 1.
It's a valid sev/sev-es policy though.

>b) Start a confidential SEV-SNP guest with policy set in command line and with this IGVM.
>I think with your patch this will fail as the policies won’t match.

Correct: this was suggested by Gerd, as this is something unexpected. I
think it would break launch measurement.

>This potentially breaks some tests. For example, with your patch, does my FUKI confidential tests still pass?

To be fair, guest policy set via igvm _never_ worked. Only the policies
set from CLI worked.

That said, I'll try to run your tests.

Luigi