arm64: kexec_file: add kernel signature verification support

With this patch, kernel verification can be done without IMA security
subsystem enabled. Turn on CONFIG_KEXEC_VERIFY_SIG instead.

On x86, a signature is embedded into a PE file (Microsoft's format) header
of binary. Since arm64's "Image" can also be seen as a PE file as far as
CONFIG_EFI is enabled, we adopt this format for kernel signing.

You can create a signed kernel image with:
    $ sbsign --key ${KEY} --cert ${CERT} Image
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Reviewed-by: James Morse <james.morse@arm.com>
[will: removed useless pr_debug()]
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/Kconfig

@@ -867,6 +867,30 @@ config KEXEC_FILE
 	  for kernel and initramfs as opposed to list of segments as
 	  accepted by previous system call.
 
+config KEXEC_VERIFY_SIG
+	bool "Verify kernel signature during kexec_file_load() syscall"
+	depends on KEXEC_FILE
+	help
+	  Select this option to verify a signature with loaded kernel
+	  image. If configured, any attempt of loading a image without
+	  valid signature will fail.
+
+	  In addition to that option, you need to enable signature
+	  verification for the corresponding kernel image type being
+	  loaded in order for this to work.
+
+config KEXEC_IMAGE_VERIFY_SIG
+	bool "Enable Image signature verification support"
+	default y
+	depends on KEXEC_VERIFY_SIG
+	depends on EFI && SIGNED_PE_FILE_VERIFICATION
+	help
+	  Enable Image signature verification support.
+
+comment "Support for PE file signature verification disabled"
+	depends on KEXEC_VERIFY_SIG
+	depends on !EFI || !SIGNED_PE_FILE_VERIFICATION
+
 config CRASH_DUMP
 	bool "Build kdump crash kernel"
 	help

arch/arm64/kernel/kexec_image.c

@@ -12,7 +12,9 @@
 #include <linux/errno.h>
 #include <linux/kernel.h>
 #include <linux/kexec.h>
+#include <linux/pe.h>
 #include <linux/string.h>
+#include <linux/verification.h>
 #include <asm/byteorder.h>
 #include <asm/cpufeature.h>
 #include <asm/image.h>
@@ -20,13 +22,13 @@
 
 static int image_probe(const char *kernel_buf, unsigned long kernel_len)
 {
-	const struct arm64_image_header *h;
+	const struct arm64_image_header *h =
+		(const struct arm64_image_header *)(kernel_buf);
 
-	h = (const struct arm64_image_header *)(kernel_buf);
+	if (!h || (kernel_len < sizeof(*h)))
+		return -EINVAL;
 
-	if (!h || (kernel_len < sizeof(*h)) ||
-			memcmp(&h->magic, ARM64_IMAGE_MAGIC,
-				sizeof(h->magic)))
+	if (memcmp(&h->magic, ARM64_IMAGE_MAGIC, sizeof(h->magic)))
 		return -EINVAL;
 
 	return 0;
@@ -107,7 +109,18 @@ static void *image_load(struct kimage *image,
 	return ERR_PTR(ret);
 }
 
+#ifdef CONFIG_KEXEC_IMAGE_VERIFY_SIG
+static int image_verify_sig(const char *kernel, unsigned long kernel_len)
+{
+	return verify_pefile_signature(kernel, kernel_len, NULL,
+				       VERIFYING_KEXEC_PE_SIGNATURE);
+}
+#endif
+
 const struct kexec_file_ops kexec_image_ops = {
 	.probe = image_probe,
 	.load = image_load,
+#ifdef CONFIG_KEXEC_IMAGE_VERIFY_SIG
+	.verify_sig = image_verify_sig,
+#endif
 };