Re: [PATCH v8 29/40] x86/compressed/64: add support for SEV-SNP CPUID table in #VC handlers

Borislav Petkov
2022-01-13T13:16:09Z

On Fri, Dec 10, 2021 at 09:43:21AM -0600, Brijesh Singh wrote:
> +/*
> + * Individual entries of the SEV-SNP CPUID table, as defined by the SEV-SNP
> + * Firmware ABI, Revision 0.9, Section 7.1, Table 14. Note that the XCR0_IN
> + * and XSS_IN are denoted here as __unused/__unused2, since they are not
> + * needed for the current guest implementation,

That's fine and great but you need to check in the function where you
iterate over those leafs below whether those unused variables are 0
and fail if not. Not that BIOS or whoever creates that table, starts
becoming creative...

> where the size of the buffers
> + * needed to store enabled XSAVE-saved features are calculated rather than
> + * encoded in the CPUID table for each possible combination of XCR0_IN/XSS_IN
> + * to save space.
> + */
> +struct snp_cpuid_fn {
> + u32 eax_in;
> + u32 ecx_in;
> + u64 __unused;
> + u64 __unused2;
> + u32 eax;
> + u32 ebx;
> + u32 ecx;
> + u32 edx;
> + u64 __reserved;

Ditto.

> +} __packed;
> +
> +/*
> + * SEV-SNP CPUID table header, as defined by the SEV-SNP Firmware ABI,
> + * Revision 0.9, Section 8.14.2.6. Also noted there is the SEV-SNP
> + * firmware-enforced limit of 64 entries per CPUID table.
> + */
> +#define SNP_CPUID_COUNT_MAX 64
> +
> +struct snp_cpuid_info {
> + u32 count;
> + u32 __reserved1;
> + u64 __reserved2;
> + struct snp_cpuid_fn fn[SNP_CPUID_COUNT_MAX];
> +} __packed;
> +
> /*
> * Since feature negotiation related variables are set early in the boot
> * process they must reside in the .data section so as not to be zeroed
> @@ -23,6 +58,20 @@
> */
> static u16 ghcb_version __ro_after_init;
>
> +/* Copy of the SNP firmware's CPUID page. */
> +static struct snp_cpuid_info cpuid_info_copy __ro_after_init;
> +static bool snp_cpuid_initialized __ro_after_init;
> +
> +/*
> + * These will be initialized based on CPUID table so that non-present
> + * all-zero leaves (for sparse tables) can be differentiated from
> + * invalid/out-of-range leaves. This is needed since all-zero leaves
> + * still need to be post-processed.
> + */
> +u32 cpuid_std_range_max __ro_after_init;
> +u32 cpuid_hyp_range_max __ro_after_init;
> +u32 cpuid_ext_range_max __ro_after_init;

All of them: static.

> static bool __init sev_es_check_cpu_features(void)
> {
> if (!has_cpuflag(X86_FEATURE_RDRAND)) {
> @@ -246,6 +295,244 @@ static int sev_cpuid_hv(u32 func, u32 subfunc, u32 *eax, u32 *ebx,
> return 0;
> }
>
> +static const struct snp_cpuid_info *

No need for that linebreak here.

> +snp_cpuid_info_get_ptr(void)
> +{
> + void *ptr;
> +
> + /*
> + * This may be called early while still running on the initial identity
> + * mapping. Use RIP-relative addressing to obtain the correct address
> + * in both for identity mapping and after switch-over to kernel virtual
> + * addresses.
> + */

Put that comment over the function name.

And yah, that probably works but eww.

> + asm ("lea cpuid_info_copy(%%rip), %0"
> + : "=r" (ptr)

Why not "=g" and let the compiler decide?

> + : "p" (&cpuid_info_copy));
> +
> + return ptr;
> +}
> +
> +static inline bool snp_cpuid_active(void)
> +{
> + return snp_cpuid_initialized;
> +}

That looks useless. That variable snp_cpuid_initialized either gets set
or the guest terminates, so practically, if the guest is still running,
you can assume SNP CPUID is properly initialized.

> +static int snp_cpuid_calc_xsave_size(u64 xfeatures_en, u32 base_size,
> + u32 *xsave_size, bool compacted)
> +{
> + const struct snp_cpuid_info *cpuid_info = snp_cpuid_info_get_ptr();
> + u32 xsave_size_total = base_size;
> + u64 xfeatures_found = 0;
> + int i;
> +
> + for (i = 0; i < cpuid_info->count; i++) {
> + const struct snp_cpuid_fn *fn = &cpuid_info->fn[i];
> +
> + if (!(fn->eax_in == 0xD && fn->ecx_in > 1 && fn->ecx_in < 64))
> + continue;

I guess that test can be as simple as

if (fn->eax_in != 0xd)
continue;

or why do you wanna check ECX too? Funky values coming from the CPUID
page?

> + if (!(xfeatures_en & (BIT_ULL(fn->ecx_in))))
> + continue;
> + if (xfeatures_found & (BIT_ULL(fn->ecx_in)))
> + continue;

What is that test for? Don't tell me the CPUID page allows duplicate
entries...

> + xfeatures_found |= (BIT_ULL(fn->ecx_in));
> +
> + if (compacted)
> + xsave_size_total += fn->eax;
> + else
> + xsave_size_total = max(xsave_size_total,
> + fn->eax + fn->ebx);
> + }
> +
> + /*
> + * Either the guest set unsupported XCR0/XSS bits, or the corresponding
> + * entries in the CPUID table were not present. This is not a valid
> + * state to be in.
> + */
> + if (xfeatures_found != (xfeatures_en & GENMASK_ULL(63, 2)))
> + return -EINVAL;
> +
> + *xsave_size = xsave_size_total;
> +
> + return 0;

This function can return xsave_size in the success case and negative in
the error case so you don't need the IO param *xsave_size.

> +}
> +
> +static void snp_cpuid_hv(u32 func, u32 subfunc, u32 *eax, u32 *ebx, u32 *ecx,
> + u32 *edx)
> +{
> + /*
> + * MSR protocol does not support fetching indexed subfunction, but is
> + * sufficient to handle current fallback cases. Should that change,
> + * make sure to terminate rather than ignoring the index and grabbing
> + * random values. If this issue arises in the future, handling can be
> + * added here to use GHCB-page protocol for cases that occur late
> + * enough in boot that GHCB page is available.
> + */
> + if (cpuid_function_is_indexed(func) && subfunc)
> + sev_es_terminate(1, GHCB_TERM_CPUID_HV);
> +
> + if (sev_cpuid_hv(func, 0, eax, ebx, ecx, edx))
> + sev_es_terminate(1, GHCB_TERM_CPUID_HV);
> +}
> +
> +static bool
> +snp_cpuid_find_validated_func(u32 func, u32 subfunc, u32 *eax, u32 *ebx,

snp_cpuid_get_validated_func()

> + u32 *ecx, u32 *edx)
> +{
> + 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 != func)
> + continue;
> +
> + if (cpuid_function_is_indexed(func) && fn->ecx_in != subfunc)
> + continue;
> +
> + *eax = fn->eax;
> + *ebx = fn->ebx;
> + *ecx = fn->ecx;
> + *edx = fn->edx;
> +
> + return true;
> + }
> +
> + return false;
> +}
> +
> +static bool snp_cpuid_check_range(u32 func)
> +{
> + if (func <= cpuid_std_range_max ||
> + (func >= 0x40000000 && func <= cpuid_hyp_range_max) ||
> + (func >= 0x80000000 && func <= cpuid_ext_range_max))
> + return true;
> +
> + return false;
> +}
> +
> +static int snp_cpuid_postprocess(u32 func, u32 subfunc, u32 *eax, u32 *ebx,
> + u32 *ecx, u32 *edx)

I'm wondering if you could make everything a lot easier by doing

static int snp_cpuid_postprocess(struct cpuid_leaf *leaf)

and marshall around that struct cpuid_leaf which contains func, subfunc,
e[abcd]x instead of dealing with 6 parameters.

Callers of snp_cpuid() can simply allocate it on their stack and hand it
in and it is all in sev-shared.c so nicely self-contained...

...

> +/*
> + * Returns -EOPNOTSUPP if feature not enabled. Any other return value should be
> + * treated as fatal by caller.
> + */
> +static int snp_cpuid(u32 func, u32 subfunc, u32 *eax, u32 *ebx, u32 *ecx,
> + u32 *edx)
> +{
> + if (!snp_cpuid_active())
> + return -EOPNOTSUPP;

And this becomes superfluous.

> +
> + if (!snp_cpuid_find_validated_func(func, subfunc, eax, ebx, ecx, edx)) {
> + /*
> + * Some hypervisors will avoid keeping track of CPUID entries
> + * where all values are zero, since they can be handled the
> + * same as out-of-range values (all-zero). This is useful here
> + * as well as it allows virtually all guest configurations to
> + * work using a single SEV-SNP CPUID table.
> + *
> + * To allow for this, there is a need to distinguish between
> + * out-of-range entries and in-range zero entries, since the
> + * CPUID table entries are only a template that may need to be
> + * augmented with additional values for things like
> + * CPU-specific information during post-processing. So if it's
> + * not in the table, but is still in the valid range, proceed
> + * with the post-processing. Otherwise, just return zeros.
> + */
> + *eax = *ebx = *ecx = *edx = 0;
> + if (!snp_cpuid_check_range(func))
> + return 0;

Do the check first and then assign.

> + }
> +
> + return snp_cpuid_postprocess(func, subfunc, eax, ebx, ecx, edx);
> +}
> +
> /*
> * Boot VC Handler - This is the first VC handler during boot, there is no GHCB
> * page yet, so it only supports the MSR based communication with the
> @@ -253,16 +540,26 @@ static int sev_cpuid_hv(u32 func, u32 subfunc, u32 *eax, u32 *ebx,
> */
> void __init do_vc_no_ghcb(struct pt_regs *regs, unsigned long exit_code)
> {
> + unsigned int subfn = lower_bits(regs->cx, 32);
> unsigned int fn = lower_bits(regs->ax, 32);
> u32 eax, ebx, ecx, edx;
> + int ret;
>
> /* Only CPUID is supported via MSR protocol */
> if (exit_code != SVM_EXIT_CPUID)
> goto fail;
>
> + ret = snp_cpuid(fn, subfn, &eax, &ebx, &ecx, &edx);
> + if (ret == 0)

if (!ret)

> + goto cpuid_done;
> +
> + if (ret != -EOPNOTSUPP)
> + goto fail;
> +
> if (sev_cpuid_hv(fn, 0, &eax, &ebx, &ecx, &edx))
> goto fail;
>
> +cpuid_done:
> regs->ax = eax;
> regs->bx = ebx;
> regs->cx = ecx;
> @@ -557,12 +854,35 @@ static enum es_result vc_handle_ioio(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
> return ret;
> }
>
> +static int vc_handle_cpuid_snp(struct pt_regs *regs)
> +{
> + u32 eax, ebx, ecx, edx;
> + int ret;
> +
> + ret = snp_cpuid(regs->ax, regs->cx, &eax, &ebx, &ecx, &edx);
> + if (ret == 0) {

if (!ret)

> + regs->ax = eax;
> + regs->bx = ebx;
> + regs->cx = ecx;
> + regs->dx = edx;
> + }
> +
> + return ret;
> +}
> +
> static enum es_result vc_handle_cpuid(struct ghcb *ghcb,
> struct es_em_ctxt *ctxt)
> {
> struct pt_regs *regs = ctxt->regs;
> u32 cr4 = native_read_cr4();
> enum es_result ret;
> + int snp_cpuid_ret;
> +
> + snp_cpuid_ret = vc_handle_cpuid_snp(regs);
> + if (snp_cpuid_ret == 0)

if (! ... - you get the idea.

--
Regards/Gruss,
Boris.