s390/kexec_file: Signature verification prototype

Add kernel signature verification to kexec_file. The verification is based
on module signature verification and works with kernel images signed via
scripts/sign-file.
Signed-off-by: Philipp Rudo <prudo@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/Kconfig

@@ -553,6 +553,17 @@ config ARCH_HAS_KEXEC_PURGATORY
 	def_bool y
 	depends on KEXEC_FILE
 
+config KEXEC_VERIFY_SIG
+	bool "Verify kernel signature during kexec_file_load() syscall"
+	depends on KEXEC_FILE && SYSTEM_DATA_VERIFICATION
+	help
+	  This option makes kernel signature verification mandatory for
+	  the kexec_file_load() syscall.
+
+	  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 ARCH_RANDOM
 	def_bool y
 	prompt "s390 architectural random number generation API"

arch/s390/configs/debug_defconfig

@@ -64,6 +64,7 @@ CONFIG_NUMA=y
 CONFIG_PREEMPT=y
 CONFIG_HZ_100=y
 CONFIG_KEXEC_FILE=y
+CONFIG_KEXEC_VERIFY_SIG=y
 CONFIG_EXPOLINE=y
 CONFIG_EXPOLINE_AUTO=y
 CONFIG_MEMORY_HOTPLUG=y

arch/s390/configs/performance_defconfig

@@ -65,6 +65,7 @@ CONFIG_NR_CPUS=512
 CONFIG_NUMA=y
 CONFIG_HZ_100=y
 CONFIG_KEXEC_FILE=y
+CONFIG_KEXEC_VERIFY_SIG=y
 CONFIG_EXPOLINE=y
 CONFIG_EXPOLINE_AUTO=y
 CONFIG_MEMORY_HOTPLUG=y

arch/s390/defconfig

@@ -39,6 +39,7 @@ CONFIG_NR_CPUS=256
 CONFIG_NUMA=y
 CONFIG_HZ_100=y
 CONFIG_KEXEC_FILE=y
+CONFIG_KEXEC_VERIFY_SIG=y
 CONFIG_CRASH_DUMP=y
 CONFIG_HIBERNATION=y
 CONFIG_PM_DEBUG=y

arch/s390/include/asm/kexec.h

@@ -65,6 +65,7 @@ struct s390_load_data {
 	size_t memsz;
 };
 
+int s390_verify_sig(const char *kernel, unsigned long kernel_len);
 void *kexec_file_add_components(struct kimage *image,
 				int (*add_kernel)(struct kimage *image,
 						  struct s390_load_data *data));

arch/s390/kernel/kexec_elf.c

@@ -125,4 +125,7 @@ static int s390_elf_probe(const char *buf, unsigned long len)
 const struct kexec_file_ops s390_kexec_elf_ops = {
 	.probe = s390_elf_probe,
 	.load = s390_elf_load,
+#ifdef CONFIG_KEXEC_VERIFY_SIG
+	.verify_sig = s390_verify_sig,
+#endif /* CONFIG_KEXEC_VERIFY_SIG */
 };

arch/s390/kernel/kexec_image.c

@@ -54,4 +54,7 @@ static int s390_image_probe(const char *buf, unsigned long len)
 const struct kexec_file_ops s390_kexec_image_ops = {
 	.probe = s390_image_probe,
 	.load = s390_image_load,
+#ifdef CONFIG_KEXEC_VERIFY_SIG
+	.verify_sig = s390_verify_sig,
+#endif /* CONFIG_KEXEC_VERIFY_SIG */
 };

arch/s390/kernel/machine_kexec_file.c

@@ -8,7 +8,11 @@
  */
 
 #include <linux/elf.h>
+#include <linux/errno.h>
 #include <linux/kexec.h>
+#include <linux/module.h>
+#include <linux/verification.h>
+#include <asm/ipl.h>
 #include <asm/setup.h>
 
 const struct kexec_file_ops * const kexec_file_loaders[] = {
@@ -17,6 +21,76 @@ const struct kexec_file_ops * const kexec_file_loaders[] = {
 	NULL,
 };
 
+#ifdef CONFIG_KEXEC_VERIFY_SIG
+/*
+ * Module signature information block.
+ *
+ * The constituents of the signature section are, in order:
+ *
+ *	- Signer's name
+ *	- Key identifier
+ *	- Signature data
+ *	- Information block
+ */
+struct module_signature {
+	u8	algo;		/* Public-key crypto algorithm [0] */
+	u8	hash;		/* Digest algorithm [0] */
+	u8	id_type;	/* Key identifier type [PKEY_ID_PKCS7] */
+	u8	signer_len;	/* Length of signer's name [0] */
+	u8	key_id_len;	/* Length of key identifier [0] */
+	u8	__pad[3];
+	__be32	sig_len;	/* Length of signature data */
+};
+
+#define PKEY_ID_PKCS7 2
+
+int s390_verify_sig(const char *kernel, unsigned long kernel_len)
+{
+	const unsigned long marker_len = sizeof(MODULE_SIG_STRING) - 1;
+	struct module_signature *ms;
+	unsigned long sig_len;
+
+	/* Skip signature verification when not secure IPLed. */
+	if (!ipl_secure_flag)
+		return 0;
+
+	if (marker_len > kernel_len)
+		return -EKEYREJECTED;
+
+	if (memcmp(kernel + kernel_len - marker_len, MODULE_SIG_STRING,
+		   marker_len))
+		return -EKEYREJECTED;
+	kernel_len -= marker_len;
+
+	ms = (void *)kernel + kernel_len - sizeof(*ms);
+	kernel_len -= sizeof(*ms);
+
+	sig_len = be32_to_cpu(ms->sig_len);
+	if (sig_len >= kernel_len)
+		return -EKEYREJECTED;
+	kernel_len -= sig_len;
+
+	if (ms->id_type != PKEY_ID_PKCS7)
+		return -EKEYREJECTED;
+
+	if (ms->algo != 0 ||
+	    ms->hash != 0 ||
+	    ms->signer_len != 0 ||
+	    ms->key_id_len != 0 ||
+	    ms->__pad[0] != 0 ||
+	    ms->__pad[1] != 0 ||
+	    ms->__pad[2] != 0) {
+		return -EBADMSG;
+	}
+
+	return verify_pkcs7_signature(kernel, kernel_len,
+				      kernel + kernel_len, sig_len,
+				      VERIFY_USE_PLATFORM_KEYRING,
+				      VERIFYING_MODULE_SIGNATURE,
+				      NULL, NULL);
+}
+#endif /* CONFIG_KEXEC_VERIFY_SIG */
+
 static int kexec_file_update_purgatory(struct kimage *image,
 				       struct s390_load_data *data)
 {