efi/libstub: implement generic EFI zboot

Implement a minimal EFI app that decompresses the real kernel image and
launches it using the firmware's LoadImage and StartImage boot services.
This removes the need for any arch-specific hacks.

Note that on systems that have UEFI secure boot policies enabled,
LoadImage/StartImage require images to be signed, or their hashes known
a priori, in order to be permitted to boot.

There are various possible strategies to work around this requirement,
but they all rely either on overriding internal PI/DXE protocols (which
are not part of the EFI spec) or omitting the firmware provided
LoadImage() and StartImage() boot services, which is also undesirable,
given that they encapsulate platform specific policies related to secure
boot and measured boot, but also related to memory permissions (whether
or not and which types of heap allocations have both write and execute
permissions.)

The only generic and truly portable way around this is to simply sign
both the inner and the outer image with the same key/cert pair, so this
is what is implemented here.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

1 files changed, 290 insertions, 0 deletions

diff --git a/drivers/firmware/efi/libstub/zboot.c b/drivers/firmware/efi/libstub/zboot.c
new file mode 100644
index 000000000000..a9f41902c908
--- /dev/null
+++ b/drivers/firmware/efi/libstub/zboot.c
@@ -0,0 +1,290 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/efi.h>
+#include <linux/pe.h>
+#include <asm/efi.h>
+#include <asm/unaligned.h>
+
+#include "efistub.h"
+
+static unsigned char zboot_heap[SZ_256K] __aligned(64);
+static unsigned long free_mem_ptr, free_mem_end_ptr;
+
+#define STATIC static
+#if defined(CONFIG_KERNEL_GZIP)
+#include "../../../../lib/decompress_inflate.c"
+#elif defined(CONFIG_KERNEL_LZ4)
+#include "../../../../lib/decompress_unlz4.c"
+#elif defined(CONFIG_KERNEL_LZMA)
+#include "../../../../lib/decompress_unlzma.c"
+#elif defined(CONFIG_KERNEL_LZO)
+#include "../../../../lib/decompress_unlzo.c"
+#elif defined(CONFIG_KERNEL_XZ)
+#undef memcpy
+#define memcpy memcpy
+#undef memmove
+#define memmove memmove
+#include "../../../../lib/decompress_unxz.c"
+#elif defined(CONFIG_KERNEL_ZSTD)
+#include "../../../../lib/decompress_unzstd.c"
+#endif
+
+extern char efi_zboot_header[];
+extern char _gzdata_start[], _gzdata_end[];
+
+static void log(efi_char16_t str[])
+{
+	efi_call_proto(efi_table_attr(efi_system_table, con_out),
+		       output_string, L"EFI decompressor: ");
+	efi_call_proto(efi_table_attr(efi_system_table, con_out),
+		       output_string, str);
+	efi_call_proto(efi_table_attr(efi_system_table, con_out),
+		       output_string, L"\n");
+}
+
+static void error(char *x)
+{
+	log(L"error() called from decompressor library\n");
+}
+
+// Local version to avoid pulling in memcmp()
+static bool guids_eq(const efi_guid_t *a, const efi_guid_t *b)
+{
+	const u32 *l = (u32 *)a;
+	const u32 *r = (u32 *)b;
+
+	return l[0] == r[0] && l[1] == r[1] && l[2] == r[2] && l[3] == r[3];
+}
+
+static efi_status_t __efiapi
+load_file(efi_load_file_protocol_t *this, efi_device_path_protocol_t *rem,
+	  bool boot_policy, unsigned long *bufsize, void *buffer)
+{
+	unsigned long compressed_size = _gzdata_end - _gzdata_start;
+	struct efi_vendor_dev_path *vendor_dp;
+	bool decompress = false;
+	unsigned long size;
+	int ret;
+
+	if (rem == NULL || bufsize == NULL)
+		return EFI_INVALID_PARAMETER;
+
+	if (boot_policy)
+		return EFI_UNSUPPORTED;
+
+	// Look for our vendor media device node in the remaining file path
+	if (rem->type == EFI_DEV_MEDIA &&
+	    rem->sub_type == EFI_DEV_MEDIA_VENDOR) {
+		vendor_dp = container_of(rem, struct efi_vendor_dev_path, header);
+		if (!guids_eq(&vendor_dp->vendorguid, &LINUX_EFI_ZBOOT_MEDIA_GUID))
+			return EFI_NOT_FOUND;
+
+		decompress = true;
+		rem = (void *)(vendor_dp + 1);
+	}
+
+	if (rem->type != EFI_DEV_END_PATH ||
+	    rem->sub_type != EFI_DEV_END_ENTIRE)
+		return EFI_NOT_FOUND;
+
+	// The uncompressed size of the payload is appended to the raw bit
+	// stream, and may therefore appear misaligned in memory
+	size = decompress ? get_unaligned_le32(_gzdata_end - 4)
+			  : compressed_size;
+	if (buffer == NULL || *bufsize < size) {
+		*bufsize = size;
+		return EFI_BUFFER_TOO_SMALL;
+	}
+
+	if (decompress) {
+		ret = __decompress(_gzdata_start, compressed_size, NULL, NULL,
+				   buffer, size, NULL, error);
+		if (ret	< 0) {
+			log(L"Decompression failed");
+			return EFI_DEVICE_ERROR;
+		}
+	} else {
+		memcpy(buffer, _gzdata_start, compressed_size);
+	}
+
+	return EFI_SUCCESS;
+}
+
+// Return the length in bytes of the device path up to the first end node.
+static int device_path_length(const efi_device_path_protocol_t *dp)
+{
+	int len = 0;
+
+	while (dp->type != EFI_DEV_END_PATH) {
+		len += dp->length;
+		dp = (void *)((u8 *)dp + dp->length);
+	}
+	return len;
+}
+
+static void append_rel_offset_node(efi_device_path_protocol_t **dp,
+				   unsigned long start, unsigned long end)
+{
+	struct efi_rel_offset_dev_path *rodp = (void *)*dp;
+
+	rodp->header.type	= EFI_DEV_MEDIA;
+	rodp->header.sub_type	= EFI_DEV_MEDIA_REL_OFFSET;
+	rodp->header.length	= sizeof(struct efi_rel_offset_dev_path);
+	rodp->reserved		= 0;
+	rodp->starting_offset	= start;
+	rodp->ending_offset	= end;
+
+	*dp = (void *)(rodp + 1);
+}
+
+static void append_ven_media_node(efi_device_path_protocol_t **dp,
+				  efi_guid_t *guid)
+{
+	struct efi_vendor_dev_path *vmdp = (void *)*dp;
+
+	vmdp->header.type	= EFI_DEV_MEDIA;
+	vmdp->header.sub_type	= EFI_DEV_MEDIA_VENDOR;
+	vmdp->header.length	= sizeof(struct efi_vendor_dev_path);
+	vmdp->vendorguid	= *guid;
+
+	*dp = (void *)(vmdp + 1);
+}
+
+static void append_end_node(efi_device_path_protocol_t **dp)
+{
+	(*dp)->type		= EFI_DEV_END_PATH;
+	(*dp)->sub_type		= EFI_DEV_END_ENTIRE;
+	(*dp)->length		= sizeof(struct efi_generic_dev_path);
+
+	++*dp;
+}
+
+asmlinkage efi_status_t __efiapi
+efi_zboot_entry(efi_handle_t handle, efi_system_table_t *systab)
+{
+	efi_device_path_protocol_t *parent_dp, *dpp, *lf2_dp, *li_dp;
+	efi_load_file2_protocol_t zboot_load_file2;
+	efi_loaded_image_t *parent, *child;
+	unsigned long exit_data_size;
+	efi_handle_t child_handle;
+	efi_handle_t zboot_handle;
+	efi_char16_t *exit_data;
+	efi_status_t status;
+	void *dp_alloc;
+	int dp_len;
+
+	WRITE_ONCE(efi_system_table, systab);
+
+	free_mem_ptr = (unsigned long)&zboot_heap;
+	free_mem_end_ptr = free_mem_ptr + sizeof(zboot_heap);
+
+	exit_data = NULL;
+	exit_data_size = 0;
+
+	status = efi_bs_call(handle_protocol, handle,
+			     &LOADED_IMAGE_PROTOCOL_GUID, (void **)&parent);
+	if (status != EFI_SUCCESS) {
+		log(L"Failed to locate parent's loaded image protocol");
+		return status;
+	}
+
+	status = efi_bs_call(handle_protocol, handle,
+			     &LOADED_IMAGE_DEVICE_PATH_PROTOCOL_GUID,
+			     (void **)&parent_dp);
+	if (status != EFI_SUCCESS) {
+		log(L"Failed to locate parent's loaded image device path protocol");
+		return status;
+	}
+
+	// Allocate some pool memory for device path protocol data
+	dp_len = parent_dp ? device_path_length(parent_dp) : 0;
+	status = efi_bs_call(allocate_pool, EFI_LOADER_DATA,
+			     2 * (dp_len + sizeof(struct efi_rel_offset_dev_path) +
+			          sizeof(struct efi_generic_dev_path)) +
+			     sizeof(struct efi_vendor_dev_path),
+			     (void **)&dp_alloc);
+	if (status != EFI_SUCCESS) {
+		log(L"Failed to allocate device path pool memory");
+		return status;
+	}
+
+	// Create a device path describing the compressed payload in this image
+	// <...parent_dp...>/Offset(<start>, <end>)
+	lf2_dp = memcpy(dp_alloc, parent_dp, dp_len);
+	dpp = (void *)((u8 *)lf2_dp + dp_len);
+	append_rel_offset_node(&dpp,
+			       (unsigned long)(_gzdata_start - efi_zboot_header),
+			       (unsigned long)(_gzdata_end - efi_zboot_header - 1));
+	append_end_node(&dpp);
+
+	// Create a device path describing the decompressed payload in this image
+	// <...parent_dp...>/Offset(<start>, <end>)/VenMedia(ZBOOT_MEDIA_GUID)
+	dp_len += sizeof(struct efi_rel_offset_dev_path);
+	li_dp = memcpy(dpp, lf2_dp, dp_len);
+	dpp = (void *)((u8 *)li_dp + dp_len);
+	append_ven_media_node(&dpp, &LINUX_EFI_ZBOOT_MEDIA_GUID);
+	append_end_node(&dpp);
+
+	zboot_handle = NULL;
+	zboot_load_file2.load_file = load_file;
+	status = efi_bs_call(install_multiple_protocol_interfaces,
+			     &zboot_handle,
+			     &EFI_DEVICE_PATH_PROTOCOL_GUID, lf2_dp,
+			     &EFI_LOAD_FILE2_PROTOCOL_GUID, &zboot_load_file2,
+			     NULL);
+	if (status != EFI_SUCCESS) {
+		log(L"Failed to install LoadFile2 protocol and device path");
+		goto free_dpalloc;
+	}
+
+	status = efi_bs_call(load_image, false, handle, li_dp, NULL, 0,
+			     &child_handle);
+	if (status != EFI_SUCCESS) {
+		log(L"Failed to load image");
+		goto uninstall_lf2;
+	}
+
+	status = efi_bs_call(handle_protocol, child_handle,
+			     &LOADED_IMAGE_PROTOCOL_GUID, (void **)&child);
+	if (status != EFI_SUCCESS) {
+		log(L"Failed to locate child's loaded image protocol");
+		goto unload_image;
+	}
+
+	// Copy the kernel command line
+	child->load_options = parent->load_options;
+	child->load_options_size = parent->load_options_size;
+
+	status = efi_bs_call(start_image, child_handle, &exit_data_size,
+			     &exit_data);
+	if (status != EFI_SUCCESS) {
+		log(L"StartImage() returned with error");
+		if (exit_data_size > 0)
+			log(exit_data);
+
+		// If StartImage() returns EFI_SECURITY_VIOLATION, the image is
+		// not unloaded so we need to do it by hand.
+		if (status == EFI_SECURITY_VIOLATION)
+unload_image:
+			efi_bs_call(unload_image, child_handle);
+	}
+
+uninstall_lf2:
+	efi_bs_call(uninstall_multiple_protocol_interfaces,
+		    zboot_handle,
+		    &EFI_DEVICE_PATH_PROTOCOL_GUID, lf2_dp,
+		    &EFI_LOAD_FILE2_PROTOCOL_GUID, &zboot_load_file2,
+		    NULL);
+
+free_dpalloc:
+	efi_bs_call(free_pool, dp_alloc);
+
+	efi_bs_call(exit, handle, status, exit_data_size, exit_data);
+
+	// Free ExitData in case Exit() returned with a failure code,
+	// but return the original status code.
+	log(L"Exit() returned with failure code");
+	if (exit_data != NULL)
+		efi_bs_call(free_pool, exit_data);
+	return status;
+}