Re: [PATCH Part2 v6 06/49] x86/sev: Add helper functions for RMPUPDATE and PSMASH instruction

[Dov Murik]

Hi Ashish,

On 21/06/2022 2:02, Ashish Kalra wrote:
> From: Brijesh Singh <brijesh.singh@xxxxxxx>
> 
> The RMPUPDATE instruction writes a new RMP entry in the RMP Table. The
> hypervisor will use the instruction to add pages to the RMP table. See
> APM3 for details on the instruction operations.
> 
> The PSMASH instruction expands a 2MB RMP entry into a corresponding set of
> contiguous 4KB-Page RMP entries. The hypervisor will use this instruction
> to adjust the RMP entry without invalidating the previous RMP entry.
> 
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> ---
>  arch/x86/include/asm/sev.h | 11 ++++++
>  arch/x86/kernel/sev.c      | 72 ++++++++++++++++++++++++++++++++++++++
>  2 files changed, 83 insertions(+)
> 
> diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
> index cb16f0e5b585..6ab872311544 100644
> --- a/arch/x86/include/asm/sev.h
> +++ b/arch/x86/include/asm/sev.h
> @@ -85,7 +85,9 @@ extern bool handle_vc_boot_ghcb(struct pt_regs *regs);
>  
>  /* RMP page size */
>  #define RMP_PG_SIZE_4K			0
> +#define RMP_PG_SIZE_2M			1
>  #define RMP_TO_X86_PG_LEVEL(level)	(((level) == RMP_PG_SIZE_4K) ? PG_LEVEL_4K : PG_LEVEL_2M)
> +#define X86_TO_RMP_PG_LEVEL(level)	(((level) == PG_LEVEL_4K) ? RMP_PG_SIZE_4K : RMP_PG_SIZE_2M)
>  
>  /*
>   * The RMP entry format is not architectural. The format is defined in PPR
> @@ -126,6 +128,15 @@ struct snp_guest_platform_data {
>  	u64 secrets_gpa;
>  };
>  
> +struct rmpupdate {
> +	u64 gpa;
> +	u8 assigned;
> +	u8 pagesize;
> +	u8 immutable;
> +	u8 rsvd;
> +	u32 asid;
> +} __packed;
> +
>  #ifdef CONFIG_AMD_MEM_ENCRYPT
>  extern struct static_key_false sev_es_enable_key;
>  extern void __sev_es_ist_enter(struct pt_regs *regs);
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index 59e7ec6b0326..f6c64a722e94 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -2429,3 +2429,75 @@ int snp_lookup_rmpentry(u64 pfn, int *level)
>  	return !!rmpentry_assigned(e);
>  }
>  EXPORT_SYMBOL_GPL(snp_lookup_rmpentry);
> +
> +int psmash(u64 pfn)
> +{
> +	unsigned long paddr = pfn << PAGE_SHIFT;
> +	int ret;
> +
> +	if (!pfn_valid(pfn))
> +		return -EINVAL;
> +
> +	if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
> +		return -ENXIO;
> +
> +	/* Binutils version 2.36 supports the PSMASH mnemonic. */
> +	asm volatile(".byte 0xF3, 0x0F, 0x01, 0xFF"
> +		      : "=a"(ret)
> +		      : "a"(paddr)
> +		      : "memory", "cc");
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(psmash);
> +
> +static int rmpupdate(u64 pfn, struct rmpupdate *val)
> +{
> +	unsigned long paddr = pfn << PAGE_SHIFT;
> +	int ret;
> +
> +	if (!pfn_valid(pfn))
> +		return -EINVAL;
> +
> +	if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
> +		return -ENXIO;
> +
> +	/* Binutils version 2.36 supports the RMPUPDATE mnemonic. */
> +	asm volatile(".byte 0xF2, 0x0F, 0x01, 0xFE"
> +		     : "=a"(ret)
> +		     : "a"(paddr), "c"((unsigned long)val)
> +		     : "memory", "cc");
> +	return ret;
> +}
> +
> +int rmp_make_private(u64 pfn, u64 gpa, enum pg_level level, int asid, bool immutable)
> +{
> +	struct rmpupdate val;
> +
> +	if (!pfn_valid(pfn))
> +		return -EINVAL;
> +

Should we add more checks on the arguments?

1. asid must be > 0
2. gpa must be aligned according to 'level'
3. gpa must be below the maximal address for the guest

"Note that the guest physical address space is limited according to
CPUID Fn80000008_EAX and thus the GPAs used by the firmware in
measurement calculation are equally limited. Hypervisors should not
attempt to map pages outside of this limit."
(-SNP ABI spec page 86, section 8.17 SNP_LAUNCH_UPDATE)


But note that in patch 28 of this series we have:

+		/* Transition the VMSA page to a firmware state. */
+		ret = rmp_make_private(pfn, -1, PG_LEVEL_4K, sev->asid, true);

That (u64)(-1) value for the gpa argument violates conditions 2 and 3
from my list above.

And indeed when calculating measurements we see that the GPA value
for the VMSA pages is 0x0000FFFF_FFFFF000, and not (u64)(-1). [1] [2]

Instead of checks, we can mask the gpa argument so that rmpupdate will
get the correct value.  Not sure which approach is preferable.


[1] https://github.com/IBM/sev-snp-measure/blob/90f6e59831d20e44d03d5ee19388f624fca87291/sevsnpmeasure/gctx.py#L40
[2] https://github.com/slp/snp-digest-rs/blob/0e5a787e99069944467151101ae4db474793d657/src/main.rs#L86


-Dov


> +	memset(&val, 0, sizeof(val));
> +	val.assigned = 1;
> +	val.asid = asid;
> +	val.immutable = immutable;
> +	val.gpa = gpa;
> +	val.pagesize = X86_TO_RMP_PG_LEVEL(level);
> +
> +	return rmpupdate(pfn, &val);
> +}
> +EXPORT_SYMBOL_GPL(rmp_make_private);
> +
> +int rmp_make_shared(u64 pfn, enum pg_level level)
> +{
> +	struct rmpupdate val;
> +
> +	if (!pfn_valid(pfn))
> +		return -EINVAL;
> +
> +	memset(&val, 0, sizeof(val));
> +	val.pagesize = X86_TO_RMP_PG_LEVEL(level);
> +
> +	return rmpupdate(pfn, &val);
> +}
> +EXPORT_SYMBOL_GPL(rmp_make_shared);