Re: [PATCH Part1 v5 34/38] x86/sev: Add snp_msg_seqno() helper

[Dov Murik]

Hi Brijesh,

On 20/08/2021 18:19, Brijesh Singh wrote:
> The SNP guest request message header contains a message count. The
> message count is used while building the IV. The PSP firmware increments
> the message count by 1, and expects that next message will be using the
> incremented count. The snp_msg_seqno() helper will be used by driver to
> get the message sequence counter used in the request message header,
> and it will be automatically incremented after the request is successful.
> The incremented value is saved in the secrets page so that the kexec'ed
> kernel knows from where to begin.
> 
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> ---
>  arch/x86/kernel/sev.c     | 79 +++++++++++++++++++++++++++++++++++++++
>  include/linux/sev-guest.h | 37 ++++++++++++++++++
>  2 files changed, 116 insertions(+)
> 
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index 319a40fc57ce..f42cd5a8e7bb 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -51,6 +51,8 @@ static struct ghcb boot_ghcb_page __bss_decrypted __aligned(PAGE_SIZE);
>   */
>  static struct ghcb __initdata *boot_ghcb;
>  
> +static u64 snp_secrets_phys;
> +
>  /* #VC handler runtime per-CPU data */
>  struct sev_es_runtime_data {
>  	struct ghcb ghcb_page;
> @@ -2030,6 +2032,80 @@ bool __init handle_vc_boot_ghcb(struct pt_regs *regs)
>  		halt();
>  }
>  
> +static struct snp_secrets_page_layout *snp_map_secrets_page(void)
> +{
> +	u16 __iomem *secrets;

You never dereference 'secrets'. Maybe s/u16/void/ ?


> +
> +	if (!snp_secrets_phys || !sev_feature_enabled(SEV_SNP))
> +		return NULL;
> +
> +	secrets = ioremap_encrypted(snp_secrets_phys, PAGE_SIZE);
> +	if (!secrets)
> +		return NULL;
> +
> +	return (struct snp_secrets_page_layout *)secrets;
> +}
> +
> +static inline u64 snp_read_msg_seqno(void)
> +{
> +	struct snp_secrets_page_layout *layout;
> +	u64 count;
> +
> +	layout = snp_map_secrets_page();
> +	if (!layout)
> +		return 0;
> +
> +	/* Read the current message sequence counter from secrets pages */
> +	count = readl(&layout->os_area.msg_seqno_0);
> +
> +	iounmap(layout);
> +
> +	/* The sequence counter must begin with 1 */
> +	if (!count)
> +		return 1;
> +
> +	return count + 1;

As Borislav noted, you can remove the "if (!count) return 1" because in
that case (count==0) the "return count+1" will return exactly 1.

-Dov


> +}
> +
> +u64 snp_msg_seqno(void)
> +{
> +	u64 count = snp_read_msg_seqno();
> +
> +	if (unlikely(!count))
> +		return 0;
> +
> +	/*
> +	 * The message sequence counter for the SNP guest request is a
> +	 * 64-bit value but the version 2 of GHCB specification defines a
> +	 * 32-bit storage for the it.
> +	 */
> +	if (count >= UINT_MAX)
> +		return 0;
> +
> +	return count;
> +}
> +EXPORT_SYMBOL_GPL(snp_msg_seqno);
> +
> +static void snp_gen_msg_seqno(void)
> +{
> +	struct snp_secrets_page_layout *layout;
> +	u64 count;
> +
> +	layout = snp_map_secrets_page();
> +	if (!layout)
> +		return;
> +
> +	/*
> +	 * The counter is also incremented by the PSP, so increment it by 2
> +	 * and save in secrets page.
> +	 */
> +	count = readl(&layout->os_area.msg_seqno_0);
> +	count += 2;
> +
> +	writel(count, &layout->os_area.msg_seqno_0);
> +	iounmap(layout);
> +}
> +
>  int snp_issue_guest_request(int type, struct snp_guest_request_data *input, unsigned long *fw_err)
>  {
>  	struct ghcb_state state;
> @@ -2077,6 +2153,9 @@ int snp_issue_guest_request(int type, struct snp_guest_request_data *input, unsi
>  		ret = -EIO;
>  	}
>  
> +	/* The command was successful, increment the sequence counter */
> +	snp_gen_msg_seqno();
> +
>  e_put:
>  	__sev_put_ghcb(&state);
>  e_restore_irq:
> diff --git a/include/linux/sev-guest.h b/include/linux/sev-guest.h
> index 24dd17507789..16b6af24fda7 100644
> --- a/include/linux/sev-guest.h
> +++ b/include/linux/sev-guest.h
> @@ -20,6 +20,41 @@ enum vmgexit_type {
>  	GUEST_REQUEST_MAX
>  };
>  
> +/*
> + * The secrets page contains 96-bytes of reserved field that can be used by
> + * the guest OS. The guest OS uses the area to save the message sequence
> + * number for each VMPCK.
> + *
> + * See the GHCB spec section Secret page layout for the format for this area.
> + */
> +struct secrets_os_area {
> +	u32 msg_seqno_0;
> +	u32 msg_seqno_1;
> +	u32 msg_seqno_2;
> +	u32 msg_seqno_3;
> +	u64 ap_jump_table_pa;
> +	u8 rsvd[40];
> +	u8 guest_usage[32];
> +} __packed;
> +
> +#define VMPCK_KEY_LEN		32
> +
> +/* See the SNP spec for secrets page format */
> +struct snp_secrets_page_layout {
> +	u32 version;
> +	u32 imien	: 1,
> +	    rsvd1	: 31;
> +	u32 fms;
> +	u32 rsvd2;
> +	u8 gosvw[16];
> +	u8 vmpck0[VMPCK_KEY_LEN];
> +	u8 vmpck1[VMPCK_KEY_LEN];
> +	u8 vmpck2[VMPCK_KEY_LEN];
> +	u8 vmpck3[VMPCK_KEY_LEN];
> +	struct secrets_os_area os_area;
> +	u8 rsvd3[3840];
> +} __packed;
> +
>  /*
>   * The error code when the data_npages is too small. The error code
>   * is defined in the GHCB specification.
> @@ -36,6 +71,7 @@ struct snp_guest_request_data {
>  #ifdef CONFIG_AMD_MEM_ENCRYPT
>  int snp_issue_guest_request(int vmgexit_type, struct snp_guest_request_data *input,
>  			    unsigned long *fw_err);
> +u64 snp_msg_seqno(void);
>  #else
>  
>  static inline int snp_issue_guest_request(int type, struct snp_guest_request_data *input,
> @@ -43,6 +79,7 @@ static inline int snp_issue_guest_request(int type, struct snp_guest_request_dat
>  {
>  	return -ENODEV;
>  }
> +static inline u64 snp_msg_seqno(void) { return 0; }
>  
>  #endif /* CONFIG_AMD_MEM_ENCRYPT */
>  #endif /* __LINUX_SEV_GUEST_H__ */
>