Re: [PATCH v8 20/40] x86/sev: Use SEV-SNP AP creation to start secondary CPUs

Borislav Petkov
2021-12-31T15:36:31Z

On Fri, Dec 10, 2021 at 09:43:12AM -0600, Brijesh Singh wrote:
> diff --git a/arch/x86/include/asm/sev-common.h b/arch/x86/include/asm/sev-common.h
> index 123a96f7dff2..38c14601ae4a 100644
> --- a/arch/x86/include/asm/sev-common.h
> +++ b/arch/x86/include/asm/sev-common.h
> @@ -104,6 +104,7 @@ enum psc_op {
> (((u64)(v) & GENMASK_ULL(63, 12)) >> 12)
>
> #define GHCB_HV_FT_SNP BIT_ULL(0)
> +#define GHCB_HV_FT_SNP_AP_CREATION (BIT_ULL(1) | GHCB_HV_FT_SNP)

Why is bit 0 ORed in? Because it "Requires SEV-SNP Feature."?

You can still enforce that requirement in the test though.

Or all those SEV features should not be bits but masks -
GHCB_HV_FT_SNP_AP_CREATION_MASK for example, seeing how the others
require the previous bits to be set too.

...

> static DEFINE_PER_CPU(struct sev_es_runtime_data*, runtime_data);
> DEFINE_STATIC_KEY_FALSE(sev_es_enable_key);
>
> +static DEFINE_PER_CPU(struct sev_es_save_area *, snp_vmsa);

This is what I mean: the struct is called "sev_es... " but the variable
"snp_...". I.e., it is all sev_<something>.

> +
> static __always_inline bool on_vc_stack(struct pt_regs *regs)
> {
> unsigned long sp = regs->sp;
> @@ -814,6 +818,231 @@ void snp_set_memory_private(unsigned long vaddr, unsigned int npages)
> pvalidate_pages(vaddr, npages, 1);
> }
>
> +static int snp_set_vmsa(void *va, bool vmsa)
> +{
> + u64 attrs;
> +
> + /*
> + * The RMPADJUST instruction is used to set or clear the VMSA bit for
> + * a page. A change to the VMSA bit is only performed when running
> + * at VMPL0 and is ignored at other VMPL levels. If too low of a target

What does "too low" mean here exactly?

The kernel is not at VMPL0 but the specified level is lower? Weird...

> + * VMPL level is specified, the instruction can succeed without changing
> + * the VMSA bit should the kernel not be in VMPL0. Using a target VMPL
> + * level of 1 will return a FAIL_PERMISSION error if the kernel is not
> + * at VMPL0, thus ensuring that the VMSA bit has been properly set when
> + * no error is returned.

We do check whether we run at VMPL0 earlier when starting the guest -
see enforce_vmpl0().

I don't think you need any of that additional verification here - just
assume you are at VMPL0.

> + */
> + attrs = 1;
> + if (vmsa)
> + attrs |= RMPADJUST_VMSA_PAGE_BIT;
> +
> + return rmpadjust((unsigned long)va, RMP_PG_SIZE_4K, attrs);
> +}
> +
> +#define __ATTR_BASE (SVM_SELECTOR_P_MASK | SVM_SELECTOR_S_MASK)
> +#define INIT_CS_ATTRIBS (__ATTR_BASE | SVM_SELECTOR_READ_MASK | SVM_SELECTOR_CODE_MASK)
> +#define INIT_DS_ATTRIBS (__ATTR_BASE | SVM_SELECTOR_WRITE_MASK)
> +
> +#define INIT_LDTR_ATTRIBS (SVM_SELECTOR_P_MASK | 2)
> +#define INIT_TR_ATTRIBS (SVM_SELECTOR_P_MASK | 3)
> +
> +static void *snp_safe_alloc_page(void)

safe?

And you don't need to say "safe" - snp_alloc_vmsa_page() is perfectly fine.

> +{
> + unsigned long pfn;
> + struct page *p;
> +
> + /*
> + * Allocate an SNP safe page to workaround the SNP erratum where
> + * the CPU will incorrectly signal an RMP violation #PF if a
> + * hugepage (2mb or 1gb) collides with the RMP entry of VMSA page.

2MB or 1GB

Collides how? The 4K frame is inside the hugepage?

> + * The recommeded workaround is to not use the large page.

Unknown word [recommeded] in comment, suggestions:
['recommended', 'recommend', 'recommitted', 'commended', 'commandeered']

> + *
> + * Allocate one extra page, use a page which is not 2mb aligned

2MB-aligned

> + * and free the other.
> + */
> + p = alloc_pages(GFP_KERNEL_ACCOUNT | __GFP_ZERO, 1);
> + if (!p)
> + return NULL;
> +
> + split_page(p, 1);
> +
> + pfn = page_to_pfn(p);
> + if (IS_ALIGNED(__pfn_to_phys(pfn), PMD_SIZE)) {
> + pfn++;
> + __free_page(p);
> + } else {
> + __free_page(pfn_to_page(pfn + 1));
> + }

AFAICT, this is doing all this stuff so that you can make sure you get a
non-2M-aligned page. I wonder if there's a way to simply ask mm to give
you such page directly.

vbabka?

> +
> + return page_address(pfn_to_page(pfn));
> +}
> +
> +static int wakeup_cpu_via_vmgexit(int apic_id, unsigned long start_ip)
> +{
> + struct sev_es_save_area *cur_vmsa, *vmsa;
> + struct ghcb_state state;
> + unsigned long flags;
> + struct ghcb *ghcb;
> + int cpu, err, ret;
> + u8 sipi_vector;
> + u64 cr4;
> +
> + if ((sev_hv_features & GHCB_HV_FT_SNP_AP_CREATION) != GHCB_HV_FT_SNP_AP_CREATION)
> + return -EOPNOTSUPP;
> +
> + /*
> + * Verify the desired start IP against the known trampoline start IP
> + * to catch any future new trampolines that may be introduced that
> + * would require a new protected guest entry point.
> + */
> + if (WARN_ONCE(start_ip != real_mode_header->trampoline_start,
> + "Unsupported SEV-SNP start_ip: %lx\n", start_ip))
> + return -EINVAL;
> +
> + /* Override start_ip with known protected guest start IP */
> + start_ip = real_mode_header->sev_es_trampoline_start;
> +
> + /* Find the logical CPU for the APIC ID */
> + for_each_present_cpu(cpu) {
> + if (arch_match_cpu_phys_id(cpu, apic_id))
> + break;
> + }
> + if (cpu >= nr_cpu_ids)
> + return -EINVAL;
> +
> + cur_vmsa = per_cpu(snp_vmsa, cpu);
> +
> + /*
> + * A new VMSA is created each time because there is no guarantee that
> + * the current VMSA is the kernels or that the vCPU is not running. If

kernel's.

And if it is not the kernel's, whose it is?

> + * an attempt was done to use the current VMSA with a running vCPU, a
> + * #VMEXIT of that vCPU would wipe out all of the settings being done
> + * here.

I don't understand - this is waking up a CPU, how can it ever be a
running vCPU which is using the current VMSA?!

There is per_cpu(snp_vmsa, cpu), who else can be using that one currently?

> + */
> + vmsa = (struct sev_es_save_area *)snp_safe_alloc_page();
> + if (!vmsa)
> + return -ENOMEM;
> +
> + /* CR4 should maintain the MCE value */
> + cr4 = native_read_cr4() & X86_CR4_MCE;
> +
> + /* Set the CS value based on the start_ip converted to a SIPI vector */
> + sipi_vector = (start_ip >> 12);
> + vmsa->cs.base = sipi_vector << 12;
> + vmsa->cs.limit = 0xffff;
> + vmsa->cs.attrib = INIT_CS_ATTRIBS;
> + vmsa->cs.selector = sipi_vector << 8;
> +
> + /* Set the RIP value based on start_ip */
> + vmsa->rip = start_ip & 0xfff;
> +
> + /* Set VMSA entries to the INIT values as documented in the APM */
> + vmsa->ds.limit = 0xffff;
> + vmsa->ds.attrib = INIT_DS_ATTRIBS;
> + vmsa->es = vmsa->ds;
> + vmsa->fs = vmsa->ds;
> + vmsa->gs = vmsa->ds;
> + vmsa->ss = vmsa->ds;
> +
> + vmsa->gdtr.limit = 0xffff;
> + vmsa->ldtr.limit = 0xffff;
> + vmsa->ldtr.attrib = INIT_LDTR_ATTRIBS;
> + vmsa->idtr.limit = 0xffff;
> + vmsa->tr.limit = 0xffff;
> + vmsa->tr.attrib = INIT_TR_ATTRIBS;
> +
> + vmsa->efer = 0x1000; /* Must set SVME bit */

verify_comment_style: Warning: No tail comments please:
arch/x86/kernel/sev.c:954 [+ vmsa->efer = 0x1000; /* Must set SVME bit */]

> + vmsa->cr4 = cr4;
> + vmsa->cr0 = 0x60000010;
> + vmsa->dr7 = 0x400;
> + vmsa->dr6 = 0xffff0ff0;
> + vmsa->rflags = 0x2;
> + vmsa->g_pat = 0x0007040600070406ULL;
> + vmsa->xcr0 = 0x1;
> + vmsa->mxcsr = 0x1f80;
> + vmsa->x87_ftw = 0x5555;
> + vmsa->x87_fcw = 0x0040;

Yah, those definitely need macros or at least comments ontop denoting
what those naked values are.

> +
> + /*
> + * Set the SNP-specific fields for this VMSA:
> + * VMPL level
> + * SEV_FEATURES (matches the SEV STATUS MSR right shifted 2 bits)
> + */

Like this^^

> + vmsa->vmpl = 0;
> + vmsa->sev_features = sev_status >> 2;
> +
> + /* Switch the page over to a VMSA page now that it is initialized */
> + ret = snp_set_vmsa(vmsa, true);
> + if (ret) {
> + pr_err("set VMSA page failed (%u)\n", ret);
> + free_page((unsigned long)vmsa);
> +
> + return -EINVAL;
> + }
> +
> + /* Issue VMGEXIT AP Creation NAE event */
> + local_irq_save(flags);
> +
> + ghcb = __sev_get_ghcb(&state);
> +
> + vc_ghcb_invalidate(ghcb);
> + ghcb_set_rax(ghcb, vmsa->sev_features);
> + ghcb_set_sw_exit_code(ghcb, SVM_VMGEXIT_AP_CREATION);
> + ghcb_set_sw_exit_info_1(ghcb, ((u64)apic_id << 32) | SVM_VMGEXIT_AP_CREATE);
> + ghcb_set_sw_exit_info_2(ghcb, __pa(vmsa));
> +
> + sev_es_wr_ghcb_msr(__pa(ghcb));
> + VMGEXIT();
> +
> + if (!ghcb_sw_exit_info_1_is_valid(ghcb) ||
> + lower_32_bits(ghcb->save.sw_exit_info_1)) {
> + pr_alert("SNP AP Creation error\n");

alert?

> + ret = -EINVAL;
> + }
> +
> + __sev_put_ghcb(&state);
> +
> + local_irq_restore(flags);
> +
> + /* Perform cleanup if there was an error */
> + if (ret) {
> + err = snp_set_vmsa(vmsa, false);
> + if (err)
> + pr_err("clear VMSA page failed (%u), leaking page\n", err);
> + else
> + free_page((unsigned long)vmsa);

That...

> +
> + vmsa = NULL;
> + }
> +
> + /* Free up any previous VMSA page */
> + if (cur_vmsa) {
> + err = snp_set_vmsa(cur_vmsa, false);
> + if (err)
> + pr_err("clear VMSA page failed (%u), leaking page\n", err);
> + else
> + free_page((unsigned long)cur_vmsa);

.. and that wants to be in a common helper.

> + }
> +
> + /* Record the current VMSA page */
> + per_cpu(snp_vmsa, cpu) = vmsa;
> +
> + return ret;
> +}

--
Regards/Gruss,
Boris.