Re: [PATCH 21/70] x86/boot/compressed/64: Add function to map a page unencrypted

[David Rientjes <rientjes@google.com>]

On Thu, 19 Mar 2020, Joerg Roedel wrote:

> From: Joerg Roedel <jroedel@suse.de>
> 
> This function is needed to map the GHCB for SEV-ES guests. The GHCB is
> used for communication with the hypervisor, so its content must not be
> encrypted.
> 
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> ---
>  arch/x86/boot/compressed/ident_map_64.c | 125 ++++++++++++++++++++++++
>  arch/x86/boot/compressed/misc.h         |   1 +
>  2 files changed, 126 insertions(+)
> 
> diff --git a/arch/x86/boot/compressed/ident_map_64.c b/arch/x86/boot/compressed/ident_map_64.c
> index feb180cced28..04a5ff4bda66 100644
> --- a/arch/x86/boot/compressed/ident_map_64.c
> +++ b/arch/x86/boot/compressed/ident_map_64.c
> @@ -26,6 +26,7 @@
>  #include <asm/init.h>
>  #include <asm/pgtable.h>
>  #include <asm/trap_defs.h>
> +#include <asm/cmpxchg.h>
>  /* Use the static base for this part of the boot process */
>  #undef __PAGE_OFFSET
>  #define __PAGE_OFFSET __PAGE_OFFSET_BASE
> @@ -157,6 +158,130 @@ void initialize_identity_maps(void)
>  	write_cr3(top_level_pgt);
>  }
>  
> +static pte_t *split_large_pmd(struct x86_mapping_info *info,
> +			      pmd_t *pmdp, unsigned long __address)
> +{
> +	unsigned long page_flags;
> +	unsigned long address;
> +	pte_t *pte;
> +	pmd_t pmd;
> +	int i;
> +
> +	pte = (pte_t *)info->alloc_pgt_page(info->context);
> +	if (!pte)
> +		return NULL;
> +
> +	address     = __address & PMD_MASK;
> +	/* No large page - clear PSE flag */
> +	page_flags  = info->page_flag & ~_PAGE_PSE;
> +
> +	/* Populate the PTEs */
> +	for (i = 0; i < PTRS_PER_PMD; i++) {
> +		set_pte(&pte[i], __pte(address | page_flags));
> +		address += PAGE_SIZE;
> +	}
> +
> +	/*
> +	 * Ideally we need to clear the large PMD first and do a TLB
> +	 * flush before we write the new PMD. But the 2M range of the
> +	 * PMD might contain the code we execute and/or the stack
> +	 * we are on, so we can't do that. But that should be safe here
> +	 * because we are going from large to small mappings and we are
> +	 * also the only user of the page-table, so there is no chance
> +	 * of a TLB multihit.
> +	 */
> +	pmd = __pmd((unsigned long)pte | info->kernpg_flag);
> +	set_pmd(pmdp, pmd);
> +	/* Flush TLB to establish the new PMD */
> +	write_cr3(top_level_pgt);
> +
> +	return pte + pte_index(__address);
> +}
> +
> +static void clflush_page(unsigned long address)
> +{
> +	unsigned int flush_size;
> +	char *cl, *start, *end;
> +
> +	/*
> +	 * Hardcode cl-size to 64 - CPUID can't be used here because that might
> +	 * cause another #VC exception and the GHCB is not ready to use yet.
> +	 */
> +	flush_size = 64;
> +	start      = (char *)(address & PAGE_MASK);
> +	end        = start + PAGE_SIZE;
> +
> +	/*
> +	 * First make sure there are no pending writes on the cache-lines to
> +	 * flush.
> +	 */
> +	asm volatile("mfence" : : : "memory");
> +
> +	for (cl = start; cl != end; cl += flush_size)
> +		clflush(cl);
> +}
> +
> +static int __set_page_decrypted(struct x86_mapping_info *info,
> +				unsigned long address)
> +{
> +	unsigned long scratch, *target;
> +	pgd_t *pgdp = (pgd_t *)top_level_pgt;
> +	p4d_t *p4dp;
> +	pud_t *pudp;
> +	pmd_t *pmdp;
> +	pte_t *ptep, pte;
> +
> +	/*
> +	 * First make sure there is a PMD mapping for 'address'.
> +	 * It should already exist, but keep things generic.
> +	 *
> +	 * To map the page just read from it and fault it in if there is no
> +	 * mapping yet. add_identity_map() can't be called here because that
> +	 * would unconditionally map the address on PMD level, destroying any
> +	 * PTE-level mappings that might already exist.  Also do something
> +	 * useless with 'scratch' so the access won't be optimized away.
> +	 */
> +	target = (unsigned long *)address;
> +	scratch = *target;
> +	arch_cmpxchg(target, scratch, scratch);
> +
> +	/*
> +	 * The page is mapped at least with PMD size - so skip checks and walk
> +	 * directly to the PMD.
> +	 */
> +	p4dp = p4d_offset(pgdp, address);
> +	pudp = pud_offset(p4dp, address);
> +	pmdp = pmd_offset(pudp, address);
> +
> +	if (pmd_large(*pmdp))
> +		ptep = split_large_pmd(info, pmdp, address);
> +	else
> +		ptep = pte_offset_kernel(pmdp, address);
> +
> +	if (!ptep)
> +		return -ENOMEM;
> +
> +	/* Clear encryption flag and write new pte */
> +	pte = pte_clear_flags(*ptep, _PAGE_ENC);
> +	set_pte(ptep, pte);
> +
> +	/* Flush TLB to map the page unencrypted */
> +	write_cr3(top_level_pgt);
> +

Is there a guarantee that this flushes the tlb if cr3 == top_level_pgt 
alrady without an invlpg?

> +	/*
> +	 * Changing encryption attributes of a page requires to flush it from
> +	 * the caches.
> +	 */
> +	clflush_page(address);
> +
> +	return 0;
> +}
> +
> +int set_page_decrypted(unsigned long address)
> +{
> +	return __set_page_decrypted(&mapping_info, address);
> +}
> +
>  static void pf_error(unsigned long error_code, unsigned long address,
>  		     struct pt_regs *regs)
>  {
> diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
> index 0e3508c5c15c..42f68a858a35 100644
> --- a/arch/x86/boot/compressed/misc.h
> +++ b/arch/x86/boot/compressed/misc.h
> @@ -98,6 +98,7 @@ static inline void choose_random_location(unsigned long input,
>  #endif
>  
>  #ifdef CONFIG_X86_64
> +extern int set_page_decrypted(unsigned long address);
>  extern unsigned char _pgtable[];
>  #endif
>