Re: [PATCH v9 40/43] x86/sev: Register SEV-SNP guest request platform device

[Peter Gonda]

On Fri, Jan 28, 2022 at 10:19 AM Brijesh Singh <brijesh.singh@xxxxxxx> wrote:
>
> Version 2 of GHCB specification provides Non Automatic Exit (NAE) that can
> be used by the SEV-SNP guest to communicate with the PSP without risk from
> a malicious hypervisor who wishes to read, alter, drop or replay the
> messages sent.
>
> SNP_LAUNCH_UPDATE can insert two special pages into the guest’s memory:
> the secrets page and the CPUID page. The PSP firmware populate the contents
> of the secrets page. The secrets page contains encryption keys used by the
> guest to interact with the firmware. Because the secrets page is encrypted
> with the guest’s memory encryption key, the hypervisor cannot read the
> keys. See SEV-SNP firmware spec for further details on the secrets page
> format.
>
> Create a platform device that the SEV-SNP guest driver can bind to get the
> platform resources such as encryption key and message id to use to
> communicate with the PSP. The SEV-SNP guest driver provides a userspace
> interface to get the attestation report, key derivation, extended
> attestation report etc.
>
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> ---
>  arch/x86/include/asm/sev.h |  4 +++
>  arch/x86/kernel/sev.c      | 61 ++++++++++++++++++++++++++++++++++++++
>  2 files changed, 65 insertions(+)
>
> diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
> index 9830ee1d6ef0..ca977493eb72 100644
> --- a/arch/x86/include/asm/sev.h
> +++ b/arch/x86/include/asm/sev.h
> @@ -95,6 +95,10 @@ struct snp_req_data {
>         unsigned int data_npages;
>  };
>
> +struct snp_guest_platform_data {
> +       u64 secrets_gpa;
> +};
> +
>  #ifdef CONFIG_AMD_MEM_ENCRYPT
>  extern struct static_key_false sev_es_enable_key;
>  extern void __sev_es_ist_enter(struct pt_regs *regs);
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index 1d3ac83226fc..1e56ab00d1f4 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -19,6 +19,9 @@
>  #include <linux/kernel.h>
>  #include <linux/mm.h>
>  #include <linux/cpumask.h>
> +#include <linux/efi.h>
> +#include <linux/platform_device.h>
> +#include <linux/io.h>
>
>  #include <asm/cpu_entry_area.h>
>  #include <asm/stacktrace.h>
> @@ -34,6 +37,7 @@
>  #include <asm/cpu.h>
>  #include <asm/apic.h>
>  #include <asm/cpuid.h>
> +#include <asm/setup.h>
>
>  #define DR7_RESET_VALUE        0x400
>
> @@ -2177,3 +2181,60 @@ int snp_issue_guest_request(u64 exit_code, struct snp_req_data *input, unsigned
>         return ret;
>  }
>  EXPORT_SYMBOL_GPL(snp_issue_guest_request);
> +
> +static struct platform_device guest_req_device = {
> +       .name           = "snp-guest",
> +       .id             = -1,
> +};
> +
> +static u64 get_secrets_page(void)
> +{
> +       u64 pa_data = boot_params.cc_blob_address;
> +       struct cc_blob_sev_info info;
> +       void *map;
> +
> +       /*
> +        * The CC blob contains the address of the secrets page, check if the
> +        * blob is present.
> +        */
> +       if (!pa_data)
> +               return 0;
> +
> +       map = early_memremap(pa_data, sizeof(info));
> +       memcpy(&info, map, sizeof(info));
> +       early_memunmap(map, sizeof(info));
> +
> +       /* smoke-test the secrets page passed */
> +       if (!info.secrets_phys || info.secrets_len != PAGE_SIZE)
> +               return 0;

This seems like an error condition worth noting. If no cc_blob_address
is passed it makes sense not to log but what if the address passed
fails this smoke test, why not log?

> +
> +       return info.secrets_phys;
> +}
> +
> +static int __init init_snp_platform_device(void)
> +{
> +       struct snp_guest_platform_data data;
> +       u64 gpa;
> +
> +       if (!cc_platform_has(CC_ATTR_GUEST_SEV_SNP))
> +               return -ENODEV;
> +
> +       gpa = get_secrets_page();
> +       if (!gpa)
> +               return -ENODEV;
> +
> +       data.secrets_gpa = gpa;
> +       if (platform_device_add_data(&guest_req_device, &data, sizeof(data)))
> +               goto e_fail;
> +
> +       if (platform_device_register(&guest_req_device))
> +               goto e_fail;
> +
> +       pr_info("SNP guest platform device initialized.\n");
> +       return 0;
> +
> +e_fail:
> +       pr_err("Failed to initialize SNP guest device\n");
> +       return -ENODEV;
> +}
> +device_initcall(init_snp_platform_device);
> --
> 2.25.1
>