Re: [PATCH v8 09/40] x86/compressed: Add helper for validating pages in the decompression stage

[Venu Busireddy <venu.busireddy@oracle.com>]

On 2021-12-10 09:43:01 -0600, Brijesh Singh wrote:
> Many of the integrity guarantees of SEV-SNP are enforced through the
> Reverse Map Table (RMP). Each RMP entry contains the GPA at which a
> particular page of DRAM should be mapped. The VMs can request the
> hypervisor to add pages in the RMP table via the Page State Change VMGEXIT
> defined in the GHCB specification. Inside each RMP entry is a Validated
> flag; this flag is automatically cleared to 0 by the CPU hardware when a
> new RMP entry is created for a guest. Each VM page can be either
> validated or invalidated, as indicated by the Validated flag in the RMP
> entry. Memory access to a private page that is not validated generates
> a #VC. A VM must use PVALIDATE instruction to validate the private page
> before using it.
> 
> To maintain the security guarantee of SEV-SNP guests, when transitioning
> pages from private to shared, the guest must invalidate the pages before
> asking the hypervisor to change the page state to shared in the RMP table.
> 
> After the pages are mapped private in the page table, the guest must issue
> a page state change VMGEXIT to make the pages private in the RMP table and
> validate it.
> 
> On boot, BIOS should have validated the entire system memory. During
> the kernel decompression stage, the VC handler uses the
> set_memory_decrypted() to make the GHCB page shared (i.e clear encryption
> attribute). And while exiting from the decompression, it calls the
> set_page_encrypted() to make the page private.
> 
> Add sev_snp_set_page_{private,shared}() helper that is used by the

Since the functions being added are snp_set_page_{private,shared}(),

s/sev_snp_set_page_/snp_set_page_/

Also, s/helper that is/helpers that are/

> set_memory_{decrypt,encrypt}() to change the page state in the RMP table.

s/decrypt,encrypt/decrypted,encrypted/

> 
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> ---
>  arch/x86/boot/compressed/ident_map_64.c | 18 +++++++++-
>  arch/x86/boot/compressed/misc.h         |  4 +++
>  arch/x86/boot/compressed/sev.c          | 46 +++++++++++++++++++++++++
>  arch/x86/include/asm/sev-common.h       | 26 ++++++++++++++
>  4 files changed, 93 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/boot/compressed/ident_map_64.c b/arch/x86/boot/compressed/ident_map_64.c
> index f7213d0943b8..ef77453cc629 100644
> --- a/arch/x86/boot/compressed/ident_map_64.c
> +++ b/arch/x86/boot/compressed/ident_map_64.c
> @@ -275,15 +275,31 @@ static int set_clr_page_flags(struct x86_mapping_info *info,
>  	 * Changing encryption attributes of a page requires to flush it from
>  	 * the caches.
>  	 */
> -	if ((set | clr) & _PAGE_ENC)
> +	if ((set | clr) & _PAGE_ENC) {
>  		clflush_page(address);
>  
> +		/*
> +		 * If the encryption attribute is being cleared, then change
> +		 * the page state to shared in the RMP table.
> +		 */
> +		if (clr)

This function is also called by set_page_non_present() with clr set to
_PAGE_PRESENT. Do we want to change the page state to shared even when
the page is not present? If not, shouldn't the check be (clr & _PAGE_ENC)?

> +			snp_set_page_shared(pte_pfn(*ptep) << PAGE_SHIFT);
> +	}
> +
>  	/* Update PTE */
>  	pte = *ptep;
>  	pte = pte_set_flags(pte, set);
>  	pte = pte_clear_flags(pte, clr);
>  	set_pte(ptep, pte);
>  
> +	/*
> +	 * If the encryption attribute is being set, then change the page state to
> +	 * private in the RMP entry. The page state must be done after the PTE
> +	 * is updated.
> +	 */
> +	if (set & _PAGE_ENC)
> +		snp_set_page_private(__pa(address & PAGE_MASK));
> +
>  	/* Flush TLB after changing encryption attribute */
>  	write_cr3(top_level_pgt);
>  
> diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
> index 23e0e395084a..01cc13c12059 100644
> --- a/arch/x86/boot/compressed/misc.h
> +++ b/arch/x86/boot/compressed/misc.h
> @@ -124,6 +124,8 @@ static inline void console_init(void)
>  void sev_enable(struct boot_params *bp);
>  void sev_es_shutdown_ghcb(void);
>  extern bool sev_es_check_ghcb_fault(unsigned long address);
> +void snp_set_page_private(unsigned long paddr);
> +void snp_set_page_shared(unsigned long paddr);
>  #else
>  static inline void sev_enable(struct boot_params *bp) { }
>  static inline void sev_es_shutdown_ghcb(void) { }
> @@ -131,6 +133,8 @@ static inline bool sev_es_check_ghcb_fault(unsigned long address)
>  {
>  	return false;
>  }
> +static inline void snp_set_page_private(unsigned long paddr) { }
> +static inline void snp_set_page_shared(unsigned long paddr) { }
>  #endif
>  
>  /* acpi.c */
> diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
> index 9be369f72299..12a93acc94ba 100644
> --- a/arch/x86/boot/compressed/sev.c
> +++ b/arch/x86/boot/compressed/sev.c
> @@ -119,6 +119,52 @@ static enum es_result vc_read_mem(struct es_em_ctxt *ctxt,
>  /* Include code for early handlers */
>  #include "../../kernel/sev-shared.c"
>  
> +static inline bool sev_snp_enabled(void)
> +{
> +	return sev_status & MSR_AMD64_SEV_SNP_ENABLED;
> +}
> +
> +static void __page_state_change(unsigned long paddr, enum psc_op op)
> +{
> +	u64 val;
> +
> +	if (!sev_snp_enabled())
> +		return;
> +
> +	/*
> +	 * If private -> shared then invalidate the page before requesting the

This comment is confusing. We don't know what the present state is,
right? If we don't, shouldn't we just say:

    If the operation is SNP_PAGE_STATE_SHARED, invalidate the page before
    requesting the state change in the RMP table.

> +	 * state change in the RMP table.
> +	 */
> +	if (op == SNP_PAGE_STATE_SHARED && pvalidate(paddr, RMP_PG_SIZE_4K, 0))
> +		sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_PVALIDATE);
> +
> +	/* Issue VMGEXIT to change the page state in RMP table. */
> +	sev_es_wr_ghcb_msr(GHCB_MSR_PSC_REQ_GFN(paddr >> PAGE_SHIFT, op));
> +	VMGEXIT();
> +
> +	/* Read the response of the VMGEXIT. */
> +	val = sev_es_rd_ghcb_msr();
> +	if ((GHCB_RESP_CODE(val) != GHCB_MSR_PSC_RESP) || GHCB_MSR_PSC_RESP_VAL(val))
> +		sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_PSC);
> +
> +	/*
> +	 * Now that page is added in the RMP table, validate it so that it is
> +	 * consistent with the RMP entry.

The page is not "added", right? Shouldn't we just say:

    Validate the page so that it is consistent with the RMP entry.

Venu