x86/efi: Add GHCB mappings when SEV-ES is active

Calling down to EFI runtime services can result in the firmware
performing VMGEXIT calls. The firmware is likely to use the GHCB of the
OS (e.g., for setting EFI variables), so each GHCB in the system needs
to be identity-mapped in the EFI page tables, as unencrypted, to avoid
page faults.

Signed-off-by: Tom Lendacky <[e-mail]>
[ [e-mail]: Moved GHCB mapping loop to sev-es.c ]
Signed-off-by: Joerg Roedel <[e-mail]>
Signed-off-by: Borislav Petkov <[e-mail]>
Acked-by: Ard Biesheuvel <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]

arch/x86/boot/compressed/sev-es.c

···
  */
 #include "misc.h"
 
+#include <asm/pgtable_types.h>
 #include <asm/sev-es.h>
 #include <asm/trapnr.h>
 #include <asm/trap_pf.h>

arch/x86/include/asm/sev-es.h

···
 	if (static_branch_unlikely(&sev_es_enable_key))
 		__sev_es_nmi_complete();
 }
+extern int __init sev_es_efi_map_ghcbs(pgd_t *pgd);
 #else
 static inline void sev_es_ist_enter(struct pt_regs *regs) { }
 static inline void sev_es_ist_exit(void) { }
 static inline int sev_es_setup_ap_jump_table(struct real_mode_header *rmh) { return 0; }
 static inline void sev_es_nmi_complete(void) { }
+static inline int sev_es_efi_map_ghcbs(pgd_t *pgd) { return 0; }
 #endif
 
 #endif

arch/x86/kernel/sev-es.c

···
 	return 0;
 }
 
+/*
+ * This is needed by the OVMF UEFI firmware which will use whatever it finds in
+ * the GHCB MSR as its GHCB to talk to the hypervisor. So make sure the per-cpu
+ * runtime GHCBs used by the kernel are also mapped in the EFI page-table.
+ */
+int __init sev_es_efi_map_ghcbs(pgd_t *pgd)
+{
+	struct sev_es_runtime_data *data;
+	unsigned long address, pflags;
+	int cpu;
+	u64 pfn;
+
+	if (!sev_es_active())
+		return 0;
+
+	pflags = _PAGE_NX | _PAGE_RW;
+
+	for_each_possible_cpu(cpu) {
+		data = per_cpu(runtime_data, cpu);
+
+		address = __pa(&data->ghcb_page);
+		pfn = address >> PAGE_SHIFT;
+
+		if (kernel_map_pages_in_pgd(pgd, pfn, address, 1, pflags))
+			return 1;
+	}
+
+	return 0;
+}
+
 static enum es_result vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
 {
 	struct pt_regs *regs = ctxt->regs;

arch/x86/platform/efi/efi_64.c

···
 #include <asm/realmode.h>
 #include <asm/time.h>
 #include <asm/pgalloc.h>
+#include <asm/sev-es.h>
 
 /*
  * We allocate runtime services regions top-down, starting from -4G, i.e.
···
 	 */
 	if (kernel_map_pages_in_pgd(pgd, 0x0, 0x0, 1, pf)) {
 		pr_err("Failed to create 1:1 mapping for the first page!\n");
+		return 1;
+	}
+
+	/*
+	 * When SEV-ES is active, the GHCB as set by the kernel will be used
+	 * by firmware. Create a 1:1 unencrypted mapping for each GHCB.
+	 */
+	if (sev_es_efi_map_ghcbs(pgd)) {
+		pr_err("Failed to create 1:1 mapping for the GHCBs!\n");
 		return 1;
 	}