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

Tom Lendacky
2021-06-16T16:13:34Z

On 6/16/21 8:07 AM, Borislav Petkov wrote:
> 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.

Yup, I'll make those changes.

>
>> 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" ?

Yeah, I debated on that one. I wanted to stay close to what the APM has,
which is PERM_MASK (TARGET_PERM_MASK actually), but it does look odd. I'll
get rid of the MASK portion of PERM_MASK.

>
>> +#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. :)

Will do.

>
>> 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?

Will do.

>
> 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.

Will do.

>
>> +{
>> + 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.

Will do. The compiler should be smart enough to do the right thing, I was
just trying to show the structure of the input. But, easy enough to drop it.

>
>> + if (vmsa)
>> + attrs |= RMPADJUST_VMSA_PAGE_BIT;
>> +
>> + /* Perform RMPADJUST */
>
> Add:
>
> /* Instruction mnemonic supported in binutils versions v2.36 and later */

Will do.

>
>> + 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.

Will do.

>
>> + : "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?!

Yup, I'll make this change. And actually, the max VMPL is defined via
CPUID, so I may look at saving that off somewhere or just retrieve it here
and use that.

>
>> + * 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.

Will do.

>
>> +#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.

Yup.

>
>> +
>> + 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;

Yup.

>
>> + /* 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?

It is to catch any change in the future that adds a new trampoline start
location, that may do something different. If that happens, then this will
do the wrong thing, so it's just a safe guard.

>
>> + /* 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.

Ah, good catch. It should be set at the end of the function in place of
the "cur_vmsa = vmsa" statement.

>
>> + 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.

Will do.

>
>> + /*
>> + * 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.

Ok.

>
>> + 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.

Ok.

Thanks,
Tom

>
>> + 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;
>> +}
>