[PATCH 5/6] x86: efistub: Prefer EFI memory attributes protocol over DXE services

[Ard Biesheuvel <ardb@kernel.org>]

Currently, we rely on DXE services in some cases to clear non-execute
restrictions from page allocations that need to be executable. This is
dodgy, because DXE services are not specified by UEFI but by PI, and
they are not intended for consumption by OS loaders. However, no
alternative existed at the time.

Now, there is a new UEFI protocol that should be used instead, so if it
exists, prefer it over the DXE services calls.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 drivers/firmware/efi/libstub/x86-stub.c | 28 ++++++++++++++------
 1 file changed, 20 insertions(+), 8 deletions(-)

diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
index 7b8717cbb96a1246..ea4024a6a04e507f 100644
--- a/drivers/firmware/efi/libstub/x86-stub.c
+++ b/drivers/firmware/efi/libstub/x86-stub.c
@@ -25,6 +25,7 @@ const efi_system_table_t *efi_system_table;
 const efi_dxe_services_table_t *efi_dxe_table;
 u32 image_offset __section(".data");
 static efi_loaded_image_t *image = NULL;
+static efi_memory_attribute_protocol_t *memattr;
 
 static efi_status_t
 preserve_pci_rom_image(efi_pci_io_protocol_t *pci, struct pci_setup_rom **__rom)
@@ -221,12 +222,18 @@ adjust_memory_range_protection(unsigned long start, unsigned long size)
 	unsigned long rounded_start, rounded_end;
 	unsigned long unprotect_start, unprotect_size;
 
-	if (efi_dxe_table == NULL)
-		return;
-
 	rounded_start = rounddown(start, EFI_PAGE_SIZE);
 	rounded_end = roundup(start + size, EFI_PAGE_SIZE);
 
+	if (memattr != NULL) {
+		efi_call_proto(memattr, clear_memory_attributes, rounded_start,
+			       rounded_end - rounded_start, EFI_MEMORY_XP);
+		return;
+	}
+
+	if (efi_dxe_table == NULL)
+		return;
+
 	/*
 	 * Don't modify memory region attributes, they are
 	 * already suitable, to lower the possibility to
@@ -913,13 +920,18 @@ asmlinkage unsigned long efi_main(efi_handle_t handle,
 	if (efi_system_table->hdr.signature != EFI_SYSTEM_TABLE_SIGNATURE)
 		efi_exit(handle, EFI_INVALID_PARAMETER);
 
-	efi_dxe_table = get_efi_config_table(EFI_DXE_SERVICES_TABLE_GUID);
-	if (efi_dxe_table &&
-	    efi_dxe_table->hdr.signature != EFI_DXE_SERVICES_TABLE_SIGNATURE) {
-		efi_warn("Ignoring DXE services table: invalid signature\n");
-		efi_dxe_table = NULL;
+	if (IS_ENABLED(CONFIG_EFI_DXE_MEM_ATTRIBUTES)) {
+		efi_dxe_table = get_efi_config_table(EFI_DXE_SERVICES_TABLE_GUID);
+		if (efi_dxe_table &&
+		    efi_dxe_table->hdr.signature != EFI_DXE_SERVICES_TABLE_SIGNATURE) {
+			efi_warn("Ignoring DXE services table: invalid signature\n");
+			efi_dxe_table = NULL;
+		}
 	}
 
+	/* grab the memory attributes protocol if it exists */
+	efi_bs_call(locate_protocol, &guid, NULL, (void **)&memattr);
+
 	if (!boot_params->acpi_rsdp_addr)
 		boot_params->acpi_rsdp_addr = (unsigned long)
 				(get_efi_config_table(ACPI_20_TABLE_GUID) ?:
-- 
2.39.2