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

[Peter Gonda <pgonda@google.com>]

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On Thu, Sep 9, 2021 at 9:26 AM Brijesh Singh <brijesh.singh@amd.com> wrote:
>
>
>
> On 9/9/21 9:54 AM, Peter Gonda wrote:
> > On Fri, Aug 20, 2021 at 9:22 AM Brijesh Singh <brijesh.singh@amd.com> 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@amd.com>
> >> ---
> >>   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?
> >
>
> The sequence number should be incremented only after the command is
> successful. In this particular case the next caller should not get the
> updated sequence number.
>
> Having said so, there is a bug in current code that will cause us to
> increment the sequence number on failure. I notice it last week and have
> it fixed in v6 wip branch.
>
> int snp_issue_guest_request(....)
> {
>
>         .....
>         .....
>
>         ret = sev_es_ghcb_hv_call(ghcb, NULL, id, input->req_gpa, input->resp_gpa);
>         if (ret)
>                 goto e_put;
>
>         if (ghcb->save.sw_exit_info_2) {
>                 ...
>                 ...
>
>                 ret = -EIO;
>                 goto e_put;   /** THIS WAS MISSING */
>         }
>
>         /* The command was successful, increment the sequence counter. */
>         snp_gen_msg_seqno();
> e_put:
>         ....
> }
>
> Does this address your concern?

So the 'snp_msg_seqno()' call in 'enc_payload' will not increment the
counter, its only incremented on 'snp_gen_msg_seqno()'? If thats
correct, that addresses my first concern.

>
>
> > 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.
> >
>
> So far, the only user for the snp_msg_seqno() is the attestation driver.
> And the driver is designed to serialize the vmgexit request and thus we
> should not run into concurrence issue.

That seems a little dangerous as any module new code or out-of-tree
module could use this function thus revealing this race condition
right? Could we at least have a comment on these functions
(snp_msg_seqno and snp_gen_msg_seqno) noting this?

>
> >> +
> >> +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
> >>
> >>