Re: [PATCH v2 03/14] x86/sev: Check for the presence of an SVSM in the SNP Secrets page

[Dionna Amalie Glaze]

On Fri, Mar 8, 2024 at 10:36 AM Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
>
> During early boot phases, check for the presence of an SVSM when running
> as an SEV-SNP guest.
>
> An SVSM is present if the 64-bit value at offset 0x148 into the secrets
> page is non-zero. If an SVSM is present, save the SVSM Calling Area
> address (CAA), located at offset 0x150 into the secrets page, and set
> the VMPL level of the guest, which should be non-zero, to indicate the
> presence of an SVSM.
>
> Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> ---
>  arch/x86/boot/compressed/sev.c    | 35 ++++++++---------
>  arch/x86/include/asm/sev-common.h |  4 ++
>  arch/x86/include/asm/sev.h        | 25 +++++++++++-
>  arch/x86/kernel/sev-shared.c      | 64 +++++++++++++++++++++++++++++++
>  arch/x86/kernel/sev.c             | 16 ++++++++
>  5 files changed, 125 insertions(+), 19 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
> index bd7222aad540..f54c0ceae1cb 100644
> --- a/arch/x86/boot/compressed/sev.c
> +++ b/arch/x86/boot/compressed/sev.c
> @@ -12,6 +12,7 @@
>   */
>  #include "misc.h"
>
> +#include <linux/mm.h>
>  #include <asm/bootparam.h>
>  #include <asm/pgtable_types.h>
>  #include <asm/sev.h>
> @@ -29,6 +30,15 @@
>  static struct ghcb boot_ghcb_page __aligned(PAGE_SIZE);
>  struct ghcb *boot_ghcb;
>
> +/*
> + * SVSM related information:
> + *   When running under an SVSM, the VMPL that Linux is executing at must be
> + *   non-zero. The VMPL is therefore used to indicate the presence of an SVSM.
> + */
> +static u8 vmpl __section(".data");
> +static u64 boot_svsm_caa_pa __section(".data");
> +static struct svsm_ca *boot_svsm_caa __section(".data");
> +
>  /*
>   * Copy a version of this function here - insn-eval.c can't be used in
>   * pre-decompression code.
> @@ -335,24 +345,6 @@ void do_boot_stage2_vc(struct pt_regs *regs, unsigned long exit_code)
>                 sev_es_terminate(SEV_TERM_SET_GEN, GHCB_SEV_ES_GEN_REQ);
>  }
>
> -static bool running_at_vmpl0(void *va)
> -{
> -       u64 attrs;
> -
> -       /*
> -        * RMPADJUST modifies RMP permissions of a lesser-privileged (numerically
> -        * higher) privilege level. Here, clear the VMPL1 permission mask of the
> -        * GHCB page. If the guest is not running at VMPL0, this will fail.
> -        *
> -        * If the guest is running at VMPL0, it will succeed. Even if that operation
> -        * modifies permission bits, it is still ok to do so currently because Linux
> -        * SNP guests running at VMPL0 only run at VMPL0, so VMPL1 or higher
> -        * permission mask changes are a don't-care.
> -        */
> -       attrs = 1;
> -       return !rmpadjust((unsigned long)va, RMP_PG_SIZE_4K, attrs);
> -}
> -
>  /*
>   * SNP_FEATURES_IMPL_REQ is the mask of SNP features that will need
>   * guest side implementation for proper functioning of the guest. If any
> @@ -480,6 +472,13 @@ static bool snp_setup(struct boot_params *bp)
>          */
>         setup_cpuid_table(cc_info);
>
> +       /*
> +        * Record the SVSM Calling Area address (CAA) if the guest is not
> +        * running at VMPL0. The CA will be used to communicate with the
> +        * SVSM to perform the SVSM services.
> +        */
> +       setup_svsm_ca(cc_info);
> +
>         /*
>          * Pass run-time kernel a pointer to CC info via boot_params so EFI
>          * config table doesn't need to be searched again during early startup
> diff --git a/arch/x86/include/asm/sev-common.h b/arch/x86/include/asm/sev-common.h
> index b463fcbd4b90..68a8cdf6fd6a 100644
> --- a/arch/x86/include/asm/sev-common.h
> +++ b/arch/x86/include/asm/sev-common.h
> @@ -159,6 +159,10 @@ struct snp_psc_desc {
>  #define GHCB_TERM_NOT_VMPL0            3       /* SNP guest is not running at VMPL-0 */
>  #define GHCB_TERM_CPUID                        4       /* CPUID-validation failure */
>  #define GHCB_TERM_CPUID_HV             5       /* CPUID failure during hypervisor fallback */
> +#define GHCB_TERM_SECRETS_PAGE         6       /* Secrets page failure */
> +#define GHCB_TERM_NO_SVSM              7       /* SVSM is not advertised in the secrets page */
> +#define GHCB_TERM_SVSM_VMPL0           8       /* SVSM is present but has set VMPL to 0 */
> +#define GHCB_TERM_SVSM_CAA             9       /* SVSM is present but the CA is not page aligned */
>
>  #define GHCB_RESP_CODE(v)              ((v) & GHCB_MSR_INFO_MASK)
>
> diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
> index 9477b4053bce..891e7d9a1f66 100644
> --- a/arch/x86/include/asm/sev.h
> +++ b/arch/x86/include/asm/sev.h
> @@ -152,9 +152,32 @@ struct snp_secrets_page_layout {
>         u8 vmpck2[VMPCK_KEY_LEN];
>         u8 vmpck3[VMPCK_KEY_LEN];
>         struct secrets_os_area os_area;
> -       u8 rsvd3[3840];
> +
> +       u8 vmsa_tweak_bitmap[64];
> +
> +       /* SVSM fields */
> +       u64 svsm_base;
> +       u64 svsm_size;
> +       u64 svsm_caa;
> +       u32 svsm_max_version;
> +       u8 svsm_guest_vmpl;
> +       u8 rsvd3[3];
> +
> +       /* Remainder of page */
> +       u8 rsvd4[3744];
>  } __packed;
>
> +/*
> + * The SVSM Calling Area (CA) related structures.
> + */
> +struct svsm_ca {
> +       u8 call_pending;
> +       u8 mem_available;
> +       u8 rsvd1[6];
> +
> +       u8 svsm_buffer[PAGE_SIZE - 8];
> +};
> +
>  #ifdef CONFIG_AMD_MEM_ENCRYPT
>  extern void __sev_es_ist_enter(struct pt_regs *regs);
>  extern void __sev_es_ist_exit(void);
> diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
> index 8b04958da5e7..00bb808be44b 100644
> --- a/arch/x86/kernel/sev-shared.c
> +++ b/arch/x86/kernel/sev-shared.c
> @@ -111,6 +111,24 @@ sev_es_terminate(unsigned int set, unsigned int reason)
>                 asm volatile("hlt\n" : : : "memory");
>  }
>
> +static bool running_at_vmpl0(void *va)
> +{
> +       u64 attrs;
> +
> +       /*
> +        * RMPADJUST modifies RMP permissions of a lesser-privileged (numerically
> +        * higher) privilege level. Here, clear the VMPL1 permission mask of the
> +        * GHCB page. If the guest is not running at VMPL0, this will fail.
> +        *
> +        * If the guest is running at VMPL0, it will succeed. Even if that operation
> +        * modifies permission bits, it is still ok to do so currently because Linux
> +        * SNP guests running at VMPL0 only run at VMPL0, so VMPL1 or higher
> +        * permission mask changes are a don't-care.
> +        */
> +       attrs = 1;
> +       return !rmpadjust((unsigned long)va, RMP_PG_SIZE_4K, attrs);
> +}
> +
>  /*
>   * The hypervisor features are available from GHCB version 2 onward.
>   */
> @@ -1267,3 +1285,49 @@ static enum es_result vc_check_opcode_bytes(struct es_em_ctxt *ctxt,
>
>         return ES_UNSUPPORTED;
>  }
> +
> +/*
> + * Maintain the GPA of the SVSM Calling Area (CA) in order to utilize the SVSM
> + * services needed when not runnuing in VMPL0.

running*

otherwise lgtm

> + */
> +static void __init setup_svsm_ca(const struct cc_blob_sev_info *cc_info)
> +{
> +       struct snp_secrets_page_layout *secrets_page;
> +       u64 caa;
> +
> +       BUILD_BUG_ON(sizeof(*secrets_page) != PAGE_SIZE);
> +
> +       /*
> +        * Use __pa() since this routine is running identity mapped when
> +        * called, both by the decompressor code and the early kernel code.
> +        */
> +       if (running_at_vmpl0((void *)__pa(&boot_ghcb_page)))
> +               return;
> +
> +       /*
> +        * Not running at VMPL0, ensure everything has been properly supplied
> +        * for running under an SVSM.
> +        */
> +       if (!cc_info || !cc_info->secrets_phys || cc_info->secrets_len != PAGE_SIZE)
> +               sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_SECRETS_PAGE);
> +
> +       secrets_page = (struct snp_secrets_page_layout *)cc_info->secrets_phys;
> +       if (!secrets_page->svsm_size)
> +               sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_NO_SVSM);
> +
> +       if (!secrets_page->svsm_guest_vmpl)
> +               sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_SVSM_VMPL0);
> +
> +       vmpl = secrets_page->svsm_guest_vmpl;
> +
> +       caa = secrets_page->svsm_caa;
> +       if (!PAGE_ALIGNED(caa))
> +               sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_SVSM_CAA);
> +
> +       /*
> +        * The CA is identity mapped when this routine is called, both by the
> +        * decompressor code and the early kernel code.
> +        */
> +       boot_svsm_caa = (struct svsm_ca *)caa;
> +       boot_svsm_caa_pa = caa;
> +}
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index b59b09c2f284..64799a04feb4 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -135,6 +135,15 @@ struct ghcb_state {
>  static DEFINE_PER_CPU(struct sev_es_runtime_data*, runtime_data);
>  static DEFINE_PER_CPU(struct sev_es_save_area *, sev_vmsa);
>
> +/*
> + * SVSM related information:
> + *   When running under an SVSM, the VMPL that Linux is executing at must be
> + *   non-zero. The VMPL is therefore used to indicate the presence of an SVSM.
> + */
> +static u8 vmpl __ro_after_init;
> +static struct svsm_ca *boot_svsm_caa __ro_after_init;
> +static u64 boot_svsm_caa_pa __ro_after_init;
> +
>  struct sev_config {
>         __u64 debug             : 1,
>
> @@ -2122,6 +2131,13 @@ bool __head snp_init(struct boot_params *bp)
>
>         setup_cpuid_table(cc_info);
>
> +       /*
> +        * Record the SVSM Calling Area address (CAA) if the guest is not
> +        * running at VMPL0. The CA will be used to communicate with the
> +        * SVSM to perform the SVSM services.
> +        */
> +       setup_svsm_ca(cc_info);
> +
>         /*
>          * The CC blob will be used later to access the secrets page. Cache
>          * it here like the boot kernel does.
> --
> 2.43.2
>
>


-- 
-Dionna Glaze, PhD (she/her)