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

David Rientjes
2020-03-20T20:53:15Z

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
>