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

[Peter Gonda]

On Fri, Aug 20, 2021 at 9:22 AM Brijesh Singh <brijesh.singh@xxxxxxx> 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;
> +
> +       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;
> +}
> +
> +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);

Do we need some sort of get sequence number, then ack that sequence
number was used API? Taking your host changes in Part2 V5 as an
example. If 'snp_setup_guest_buf' fails the given sequence number is
never actually used by a message to the PSP. So the guest will have
the wrong current sequence number, an off by 1 error, right?

Also it seems like there is a concurrency error waiting to happen
here. If 2 callers call snp_msg_seqno() before either actually places
a call to the PSP, if the first caller's request doesn't reach the PSP
before the second caller's request both calls will fail. And again I
think the sequence numbers in the guest will be incorrect and
unrecoverable.

> +
> +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__ */
> --
> 2.17.1
>
>