Re: [PATCH Part1 v5 35/38] x86/sev: Register SNP guest request platform device

[Dov Murik]

Hi Brijesh,


On 20/08/2021 18:19, Brijesh Singh wrote:
> Version 2 of GHCB specification provides NAEs that can be used by the SNP
> guest to communicate with the PSP without risk from a malicious hypervisor
> who wishes to read, alter, drop or replay the messages sent.
> 
> In order to communicate with the PSP, the guest need to locate the secrets
> page inserted by the hypervisor during the SEV-SNP guest launch. The
> secrets page contains the communication keys used to send and receive the
> encrypted messages between the guest and the PSP. The secrets page location
> is passed through the setup_data.
> 
> Create a platform device that the 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 SNP guest driver can provide userspace
> interface to get the attestation report, key derivation, extended
> attestation report etc.
> 
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> ---
>  arch/x86/kernel/sev.c     | 68 +++++++++++++++++++++++++++++++++++++++
>  include/linux/sev-guest.h |  5 +++
>  2 files changed, 73 insertions(+)
> 
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index f42cd5a8e7bb..ab17c93634e9 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -22,6 +22,8 @@
>  #include <linux/log2.h>
>  #include <linux/efi.h>
>  #include <linux/sev-guest.h>
> +#include <linux/platform_device.h>
> +#include <linux/io.h>
>  
>  #include <asm/cpu_entry_area.h>
>  #include <asm/stacktrace.h>
> @@ -37,6 +39,7 @@
>  #include <asm/apic.h>
>  #include <asm/efi.h>
>  #include <asm/cpuid.h>
> +#include <asm/setup.h>
>  
>  #include "sev-internal.h"
>  
> @@ -2164,3 +2167,68 @@ int snp_issue_guest_request(int type, struct snp_guest_request_data *input, unsi
>  	return ret;
>  }
>  EXPORT_SYMBOL_GPL(snp_issue_guest_request);
> +
> +static struct platform_device guest_req_device = {
> +	.name		= "snp-guest",
> +	.id		= -1,
> +};
> +
> +static u64 find_secrets_paddr(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));
> +
> +	/* Verify that secrets page address is passed */
> +	if (info.secrets_phys && info.secrets_len == PAGE_SIZE)
> +		return info.secrets_phys;
> +
> +	return 0;
> +}
> +
> +static int __init add_snp_guest_request(void)
> +{
> +	struct snp_secrets_page_layout *layout;
> +	struct snp_guest_platform_data data;
> +
> +	if (!sev_feature_enabled(SEV_SNP))
> +		return -ENODEV;
> +
> +	snp_secrets_phys = find_secrets_paddr();
> +	if (!snp_secrets_phys)
> +		return -ENODEV;
> +
> +	layout = snp_map_secrets_page();
> +	if (!layout)
> +		return -ENODEV;
> +
> +	/*
> +	 * The secrets page contains three VMPCK that can be used for
> +	 * communicating with the PSP. We choose the VMPCK0 to encrypt guest
> +	 * messages send and receive by the Linux. Provide the key and
> +	 * id through the platform data to the driver.
> +	 */
> +	data.vmpck_id = 0;
> +	memcpy_fromio(data.vmpck, layout->vmpck0, sizeof(data.vmpck));
> +
> +	iounmap(layout);
> +
> +	platform_device_add_data(&guest_req_device, &data, sizeof(data));
> +
> +	if (!platform_device_register(&guest_req_device))
> +		dev_info(&guest_req_device.dev, "secret phys 0x%llx\n", snp_secrets_phys);

Should you return the error code from platform_device_register() in case
it fails (returns something other than zero)?

-Dov

> +
> +	return 0;
> +}
> +device_initcall(add_snp_guest_request);
> diff --git a/include/linux/sev-guest.h b/include/linux/sev-guest.h
> index 16b6af24fda7..e1cb3f7dd034 100644
> --- a/include/linux/sev-guest.h
> +++ b/include/linux/sev-guest.h
> @@ -68,6 +68,11 @@ struct snp_guest_request_data {
>  	unsigned int data_npages;
>  };
>  
> +struct snp_guest_platform_data {
> +	u8 vmpck_id;
> +	char vmpck[VMPCK_KEY_LEN];
> +};
> +
>  #ifdef CONFIG_AMD_MEM_ENCRYPT
>  int snp_issue_guest_request(int vmgexit_type, struct snp_guest_request_data *input,
>  			    unsigned long *fw_err);
>