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

Joerg Roedel
2020-03-19T09:19:03Z

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);
+
+ /*
+ * 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

--
2.17.1