Re: [PATCH v15 03/13] x86/sev: Add Secure TSC support for SNP guests

Borislav Petkov
2024-12-05T11:55:31Z

On Tue, Dec 03, 2024 at 02:30:35PM +0530, Nikunj A Dadhania wrote:
> Add support for Secure TSC in SNP-enabled guests. Secure TSC allows guests
> to securely use RDTSC/RDTSCP instructions, ensuring that the parameters
> used cannot be altered by the hypervisor once the guest is launched.
>
> Secure TSC-enabled guests need to query TSC information from the AMD
> Security Processor. This communication channel is encrypted between the AMD
> Security Processor and the guest, with the hypervisor acting merely as a
> conduit to deliver the guest messages to the AMD Security Processor. Each
> message is protected with AEAD (AES-256 GCM).
>
> Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
> Tested-by: Peter Gonda <pgonda@google.com>

> Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>

This patch changed somewhat from last time. When did Peter test it again and
Tom review it again?

> diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
> index a61898c7f114..39683101b526 100644
> --- a/arch/x86/coco/sev/core.c
> +++ b/arch/x86/coco/sev/core.c
> @@ -96,6 +96,14 @@ static u64 sev_hv_features __ro_after_init;
> /* Secrets page physical address from the CC blob */
> static u64 secrets_pa __ro_after_init;
>
> +/*
> + * For Secure TSC guests, the BP fetches TSC_INFO using SNP guest messaging and

s/BP/BSP/

> + * initializes snp_tsc_scale and snp_tsc_offset. These values are replicated
> + * across the APs VMSA fields (TSC_SCALE and TSC_OFFSET).
> + */
> +static u64 snp_tsc_scale __ro_after_init;
> +static u64 snp_tsc_offset __ro_after_init;
> +
> /* #VC handler runtime per-CPU data */
> struct sev_es_runtime_data {
> struct ghcb ghcb_page;

...

> + memcpy(tsc_resp, buf, sizeof(*tsc_resp));
> + pr_debug("%s: response status 0x%x scale 0x%llx offset 0x%llx factor 0x%x\n",
> + __func__, tsc_resp->status, tsc_resp->tsc_scale, tsc_resp->tsc_offset,
> + tsc_resp->tsc_factor);
> +
> + if (tsc_resp->status == 0) {

Like the last time:

if (!tsc_resp->status)

> + snp_tsc_scale = tsc_resp->tsc_scale;
> + snp_tsc_offset = tsc_resp->tsc_offset;
> + } else {
> + pr_err("Failed to get TSC info, response status 0x%x\n", tsc_resp->status);
> + rc = -EIO;
> + }
> +
> +e_request:
> + /* The response buffer contains sensitive data, explicitly clear it. */
> + memzero_explicit(buf, sizeof(buf));
> + memzero_explicit(tsc_resp, sizeof(*tsc_resp));
> +e_free_mdesc:
> + snp_msg_free(mdesc);
> +e_free_buf:
> + kfree(buf);
> +e_free_rio:
> + kfree(rio);
> +e_free_req:
> + kfree(req);
> + e_free_tsc_resp:
> + kfree(tsc_resp);
> +e_free_tsc_req:
> + kfree(tsc_req);
> +
> + return rc;
> +}

--
Regards/Gruss,
Boris.