Re: [PATCH Part1 RFC v3 19/22] x86/sev-snp: SEV-SNP AP creation support

Borislav Petkov
2021-06-16T13:07:47Z

On Wed, Jun 02, 2021 at 09:04:13AM -0500, Brijesh Singh wrote:
> From: Tom Lendacky <thomas.lendacky@amd.com>

> Subject: Re: [PATCH Part1 RFC v3 19/22] x86/sev-snp: SEV-SNP AP creation support

The condensed patch description in the subject line should be written in
imperative tone. I.e., it needs a verb.

And to simplify it even more, let's prefix all SEV-* stuff with
"x86/sev: " from now on to mean the whole encrypted virt area.

> To provide a more secure way to start APs under SEV-SNP, use the SEV-SNP
> AP Creation NAE event. This allows for guest control over the AP register
> state rather than trusting the hypervisor with the SEV-ES Jump Table
> address.
>
> During native_smp_prepare_cpus(), invoke an SEV-SNP function that, if
> SEV-SNP is active, will set or override apic->wakeup_secondary_cpu. This
> will allow the SEV-SNP AP Creation NAE event method to be used to boot
> the APs.
>
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> ---
> arch/x86/include/asm/sev-common.h | 1 +
> arch/x86/include/asm/sev.h | 13 ++
> arch/x86/include/uapi/asm/svm.h | 5 +
> arch/x86/kernel/sev-shared.c | 5 +
> arch/x86/kernel/sev.c | 206 ++++++++++++++++++++++++++++++
> arch/x86/kernel/smpboot.c | 3 +
> 6 files changed, 233 insertions(+)
>
> diff --git a/arch/x86/include/asm/sev-common.h b/arch/x86/include/asm/sev-common.h
> index 86bb185b5ec1..47aa57bf654a 100644
> --- a/arch/x86/include/asm/sev-common.h
> +++ b/arch/x86/include/asm/sev-common.h
> @@ -57,6 +57,7 @@
> (((unsigned long)((v) & GHCB_MSR_HV_FT_MASK) >> GHCB_MSR_HV_FT_POS))
>
> #define GHCB_HV_FT_SNP BIT_ULL(0)
> +#define GHCB_HV_FT_SNP_AP_CREATION (BIT_ULL(1) | GHCB_HV_FT_SNP)
>
> /* SNP Page State Change */
> #define GHCB_MSR_PSC_REQ 0x014
> diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
> index e2141fc28058..640108402ae9 100644
> --- a/arch/x86/include/asm/sev.h
> +++ b/arch/x86/include/asm/sev.h
> @@ -71,6 +71,13 @@ enum snp_mem_op {
> MEMORY_SHARED
> };
>
> +#define RMPADJUST_VMPL_MAX 3
> +#define RMPADJUST_VMPL_MASK GENMASK(7, 0)
> +#define RMPADJUST_VMPL_SHIFT 0
> +#define RMPADJUST_PERM_MASK_MASK GENMASK(7, 0)

mask mask huh?

How about "perm mask" and "perm shift" ?

> +#define RMPADJUST_PERM_MASK_SHIFT 8
> +#define RMPADJUST_VMSA_PAGE_BIT BIT(16)
> +
> #ifdef CONFIG_AMD_MEM_ENCRYPT
> extern struct static_key_false sev_es_enable_key;
> extern void __sev_es_ist_enter(struct pt_regs *regs);
> @@ -116,6 +123,9 @@ void __init early_snp_set_memory_shared(unsigned long vaddr, unsigned long paddr
> void __init snp_prep_memory(unsigned long paddr, unsigned int sz, int op);
> void snp_set_memory_shared(unsigned long vaddr, unsigned int npages);
> void snp_set_memory_private(unsigned long vaddr, unsigned int npages);
> +

No need for the newlines here - it is all function prototypes lumped
together - the only one who reads them is the compiler.

> +void snp_setup_wakeup_secondary_cpu(void);

"setup" "wakeup" huh?

snp_set_wakeup_secondary_cpu() looks just fine to me. :)

> diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
> index b62226bf51b9..7139c9ba59b2 100644
> --- a/arch/x86/kernel/sev-shared.c
> +++ b/arch/x86/kernel/sev-shared.c
> @@ -32,6 +32,11 @@ static bool __init sev_es_check_cpu_features(void)
> return true;
> }
>
> +static bool snp_ap_creation_supported(void)
> +{
> + return (hv_features & GHCB_HV_FT_SNP_AP_CREATION) == GHCB_HV_FT_SNP_AP_CREATION;
> +}

Can we get rid of those silly accessors pls?

We established earlier that hv_features is going to be __ro_after_init
so we might just as well export it to sev.c for direct querying -
there's no worry that something'll change it during runtime.

> static bool __init sev_snp_check_hypervisor_features(void)
> {
> if (ghcb_version < 2)
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index 4847ac81cca3..8f7ef35a25ef 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -19,6 +19,7 @@
> #include <linux/memblock.h>
> #include <linux/kernel.h>
> #include <linux/mm.h>
> +#include <linux/cpumask.h>
>
> #include <asm/cpu_entry_area.h>
> #include <asm/stacktrace.h>
> @@ -31,6 +32,7 @@
> #include <asm/svm.h>
> #include <asm/smp.h>
> #include <asm/cpu.h>
> +#include <asm/apic.h>
>
> #include "sev-internal.h"
>
> @@ -106,6 +108,8 @@ struct ghcb_state {
> 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);
> +
> /* Needed in vc_early_forward_exception */
> void do_early_exception(struct pt_regs *regs, int trapnr);
>
> @@ -744,6 +748,208 @@ void snp_set_memory_private(unsigned long vaddr, unsigned int npages)
> pvalidate_pages(vaddr, npages, 1);
> }
>
> +static int snp_rmpadjust(void *va, unsigned int vmpl, unsigned int perm_mask, bool vmsa)

No need for the "snp_" prefix. Drop it for all static functions here too
pls.

@vmpl can be a u8 so that you don't need to mask it off. The same for
@perm_mask. And then you can drop the mask defines too.

> +{
> + unsigned int attrs;
> + int err;
> +
> + attrs = (vmpl & RMPADJUST_VMPL_MASK) << RMPADJUST_VMPL_SHIFT;

Shift by 0 huh? Can we drop this silliness pls?

/* Make sure Reserved[63:17] is 0 */
attrs = 0;

attrs |= vmpl;

Plain and simple.

> + attrs |= (perm_mask & RMPADJUST_PERM_MASK_MASK) << RMPADJUST_PERM_MASK_SHIFT;

perm_mask is always 0 - you don't even have to pass it in as a function
argument.

> + if (vmsa)
> + attrs |= RMPADJUST_VMSA_PAGE_BIT;
> +
> + /* Perform RMPADJUST */

Add:

/* Instruction mnemonic supported in binutils versions v2.36 and later */

> + asm volatile (".byte 0xf3,0x0f,0x01,0xfe\n\t"
> + : "=a" (err)

here you should do:

: ... "c" (RMP_PG_SIZE_4K), ...

so that it is clear what goes into %rcx.

> + : "a" (va), "c" (0), "d" (attrs)
> + : "memory", "cc");
> +
> + return err;
> +}
> +
> +static int snp_clear_vmsa(void *vmsa)
> +{
> + /*
> + * Clear the VMSA attribute for the page:
> + * RDX[7:0] = 1, Target VMPL level, must be numerically
> + * higher than current level (VMPL0)

But RMPADJUST_VMPL_MAX is 3?!

> + * RDX[15:8] = 0, Target permission mask (not used)
> + * RDX[16] = 0, Not a VMSA page
> + */
> + return snp_rmpadjust(vmsa, RMPADJUST_VMPL_MAX, 0, false);
> +}
> +
> +static int snp_set_vmsa(void *vmsa)
> +{
> + /*
> + * To set the VMSA attribute for the page:
> + * RDX[7:0] = 1, Target VMPL level, must be numerically
> + * higher than current level (VMPL0)
> + * RDX[15:8] = 0, Target permission mask (not used)
> + * RDX[16] = 1, VMSA page
> + */
> + return snp_rmpadjust(vmsa, RMPADJUST_VMPL_MAX, 0, true);
> +}
> +
> +#define INIT_CS_ATTRIBS (SVM_SELECTOR_P_MASK | SVM_SELECTOR_S_MASK | SVM_SELECTOR_READ_MASK | SVM_SELECTOR_CODE_MASK)
> +#define INIT_DS_ATTRIBS (SVM_SELECTOR_P_MASK | SVM_SELECTOR_S_MASK | SVM_SELECTOR_WRITE_MASK)
> +

Put SVM_SELECTOR_P_MASK | SVM_SELECTOR_S_MASK in a helper define and share it in the two
definitions above pls.

> +#define INIT_LDTR_ATTRIBS (SVM_SELECTOR_P_MASK | 2)
> +#define INIT_TR_ATTRIBS (SVM_SELECTOR_P_MASK | 3)
> +
> +static int snp_wakeup_cpu_via_vmgexit(int apic_id, unsigned long start_ip)
> +{
> + struct sev_es_save_area *cur_vmsa;
> + struct sev_es_save_area *vmsa;
> + struct ghcb_state state;
> + struct ghcb *ghcb;
> + unsigned long flags;
> + u8 sipi_vector;
> + u64 cr4;
> + int cpu;
> + int ret;

Remember the reversed xmas tree. And you can combine the variables of
the same type into a single line.

> +
> + if (!snp_ap_creation_supported())
> + return -ENOTSUPP;

WARNING: ENOTSUPP is not a SUSV4 error code, prefer EOPNOTSUPP
#320: FILE: arch/x86/kernel/sev.c:813:
+ return -ENOTSUPP;

> + /* Override start_ip with known SEV-ES/SEV-SNP starting RIP */
> + if (start_ip == real_mode_header->trampoline_start) {
> + start_ip = real_mode_header->sev_es_trampoline_start;
> + } else {
> + WARN_ONCE(1, "unsupported SEV-SNP start_ip: %lx\n", start_ip);
> + return -EINVAL;
> + }

What's all that checking for? Why not simply and unconditionally doing:

start_ip = real_mode_header->sev_es_trampoline_start;

?

We are waking up an SNP guest so who cares what the previous start_ip
value was?

> + /* 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);

Where is that snp_vmsa thing used? I don't see it anywhere in the whole
patchset.

> + vmsa = (struct sev_es_save_area *)get_zeroed_page(GFP_KERNEL);
> + 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 */
> + 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;

Align them all on a single vertical line pls.

> + /*
> + * Set the SNP-specific fields for this VMSA:
> + * VMPL level
> + * SEV_FEATURES (matches the SEV STATUS MSR right shifted 2 bits)
> + */
> + 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);
> + 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_es_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");
> + ret = -EINVAL;
> + }
> +
> + sev_es_put_ghcb(&state);
> +
> + local_irq_restore(flags);
> +
> + /* Perform cleanup if there was an error */
> + if (ret) {
> + int err = snp_clear_vmsa(vmsa);
> +

^ Superfluous newline.

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

^ Superfluous newline.

> + if (err)
> + pr_err("clear VMSA page failed (%u), leaking page\n", err);
> + else
> + free_page((unsigned long)cur_vmsa);
> + }
> +
> + /* Record the current VMSA page */
> + cur_vmsa = vmsa;
> +
> + return ret;
> +}

--
Regards/Gruss,
Boris.