Re: [PATCH v8 32/40] x86/compressed: use firmware-validated CPUID for SEV-SNP guests

Borislav Petkov
2022-01-20T12:26:09Z

On Fri, Dec 10, 2021 at 09:43:24AM -0600, Brijesh Singh wrote:
> Subject: Re: [PATCH v8 32/40] x86/compressed: use firmware-validated CPUID for SEV-SNP guests
^
leafs

or so.

> From: Michael Roth <michael.roth@amd.com>
>
> SEV-SNP guests will be provided the location of special 'secrets'
> 'CPUID' pages via the Confidential Computing blob. This blob is
> provided to the boot kernel either through an EFI config table entry,
> or via a setup_data structure as defined by the Linux Boot Protocol.
>
> Locate the Confidential Computing from these sources and, if found,
> use the provided CPUID page/table address to create a copy that the
> boot kernel will use when servicing cpuid instructions via a #VC

CPUID

> handler.
>
> Signed-off-by: Michael Roth <michael.roth@amd.com>
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> ---
> arch/x86/boot/compressed/sev.c | 13 ++++++++++
> arch/x86/include/asm/sev.h | 1 +
> arch/x86/kernel/sev-shared.c | 43 ++++++++++++++++++++++++++++++++++
> 3 files changed, 57 insertions(+)
>
> diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
> index 93e125da12cf..29dfb34b5907 100644
> --- a/arch/x86/boot/compressed/sev.c
> +++ b/arch/x86/boot/compressed/sev.c
> @@ -415,6 +415,19 @@ bool snp_init(struct boot_params *bp)
> if (!cc_info)
> return false;
>
> + /*
> + * If SEV-SNP-specific Confidential Computing blob is present, then
^
a

> + * firmware/bootloader have indicated SEV-SNP support. Verifying this
> + * involves CPUID checks which will be more reliable if the SEV-SNP
> + * CPUID table is used. See comments for snp_cpuid_info_create() for

s/for/over/ ?

> + * more details.
> + */
> + snp_cpuid_info_create(cc_info);
> +
> + /* SEV-SNP CPUID table should be set up now. */
> + if (!snp_cpuid_active())
> + sev_es_terminate(1, GHCB_TERM_CPUID);

Right, that is not needed now.

> * 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.h b/arch/x86/include/asm/sev.h
> index cd189c20bcc4..4fa7ca20d7c9 100644
> --- a/arch/x86/include/asm/sev.h
> +++ b/arch/x86/include/asm/sev.h
> @@ -157,6 +157,7 @@ bool snp_init(struct boot_params *bp);
> * sev-shared.c via #include and these declarations can be dropped.
> */
> struct cc_blob_sev_info *snp_find_cc_blob_setup_data(struct boot_params *bp);
> +void snp_cpuid_info_create(const struct cc_blob_sev_info *cc_info);
> #else
> static inline void sev_es_ist_enter(struct pt_regs *regs) { }
> static inline void sev_es_ist_exit(void) { }
> diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
> index bd58a4ce29c8..5cb8f87df4b3 100644
> --- a/arch/x86/kernel/sev-shared.c
> +++ b/arch/x86/kernel/sev-shared.c
> @@ -403,6 +403,23 @@ snp_cpuid_find_validated_func(u32 func, u32 subfunc, u32 *eax, u32 *ebx,
> return false;
> }
>
> +static void __init snp_cpuid_set_ranges(void)
> +{
> + const struct snp_cpuid_info *cpuid_info = snp_cpuid_info_get_ptr();
> + int i;
> +
> + for (i = 0; i < cpuid_info->count; i++) {
> + const struct snp_cpuid_fn *fn = &cpuid_info->fn[i];
> +
> + if (fn->eax_in == 0x0)
> + cpuid_std_range_max = fn->eax;
> + else if (fn->eax_in == 0x40000000)
> + cpuid_hyp_range_max = fn->eax;
> + else if (fn->eax_in == 0x80000000)
> + cpuid_ext_range_max = fn->eax;
> + }
> +}

Kinda arbitrary to have a separate function which has a single caller.
You can just as well move the loop into snp_cpuid_info_create() and put
a comment above it:

/* Set CPUID ranges. */
for (i = 0; i < cpuid_info->count; i++) {
...

Also, snp_cpuid_info_create() should be called snp_setup_cpuid_table()
which is what this thing does.

Thx.

--
Regards/Gruss,
Boris.