[PATCH v9 10/11] arm64: kexec_file: add kernel signature verification support

[AKASHI Takahiro <takahiro.akashi@linaro.org>]

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>
---
 arch/arm64/Kconfig              | 24 ++++++++++++++++++++++++
 arch/arm64/include/asm/kexec.h  | 16 ++++++++++++++++
 arch/arm64/kernel/kexec_image.c | 15 +++++++++++++++
 3 files changed, 55 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index d8f0dcdb8b96..5c772601840d 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -857,6 +857,30 @@ 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 "Image signature verification is missing yet"
+	depends on KEXEC_VERIFY_SIG
+	depends on !EFI || !SIGNED_PE_FILE_VERIFICATION
+
 config CRASH_DUMP
 	bool "Build kdump crash kernel"
 	help
diff --git a/arch/arm64/include/asm/kexec.h b/arch/arm64/include/asm/kexec.h
index 77f05bcf6a42..891f2484969d 100644
--- a/arch/arm64/include/asm/kexec.h
+++ b/arch/arm64/include/asm/kexec.h
@@ -133,6 +133,7 @@ struct arm64_image_header {
 };
 
 static const u8 arm64_image_magic[4] = {'A', 'R', 'M', 0x64U};
+static const u8 arm64_image_pe_sig[2] = {'M', 'Z'};
 
 /**
  * arm64_header_check_magic - Helper to check the arm64 image header.
@@ -154,6 +155,21 @@ static inline int arm64_header_check_magic(const struct arm64_image_header *h)
 		&& h->magic[3] == arm64_image_magic[3]);
 }
 
+/**
+ * arm64_header_check_pe_sig - Helper to check the arm64 image header.
+ *
+ * Returns non-zero if 'MZ' signature is found.
+ */
+
+static inline int arm64_header_check_pe_sig(const struct arm64_image_header *h)
+{
+	if (!h)
+		return 0;
+
+	return (h->pe_sig[0] == arm64_image_pe_sig[0]
+		&& h->pe_sig[1] == arm64_image_pe_sig[1]);
+}
+
 extern const struct kexec_file_ops kexec_image_ops;
 
 struct kimage;
diff --git a/arch/arm64/kernel/kexec_image.c b/arch/arm64/kernel/kexec_image.c
index 2b3baf7285e0..7c11beefe65f 100644
--- a/arch/arm64/kernel/kexec_image.c
+++ b/arch/arm64/kernel/kexec_image.c
@@ -12,6 +12,7 @@
 #include <linux/errno.h>
 #include <linux/kernel.h>
 #include <linux/kexec.h>
+#include <linux/verification.h>
 #include <asm/byteorder.h>
 #include <asm/memory.h>
 
@@ -24,6 +25,9 @@ static int image_probe(const char *kernel_buf, unsigned long kernel_len)
 	if ((kernel_len < sizeof(*h)) || !arm64_header_check_magic(h))
 		return -EINVAL;
 
+	pr_debug("PE format: %s\n",
+			(arm64_header_check_pe_sig(h) ? "yes" : "no"));
+
 	return 0;
 }
 
@@ -78,7 +82,18 @@ 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
 };
-- 
2.17.0