Re: [PATCH 08/30] kexec_file: Restrict at runtime if the kernel is locked down

[David Howells]

I'm intending on inserting the attached patch before this one.

David
---
commit 87a39b258eca2e15884ee90c3fcd5758d6057b17
Author: David Howells <dhowells@xxxxxxxxxx>
Date:   Thu Feb 22 13:42:04 2018 +0000

    kexec_file: split KEXEC_VERIFY_SIG into KEXEC_SIG and KEXEC_SIG_FORCE
    
    This is a preparatory patch for kexec_file_load() lockdown.  A locked down
    kernel needs to prevent unsigned kernel images to be loaded with
    kexec_file_load().  Currently, the only way to force the signature
    verification is compiling with KEXEC_VERIFY_SIG.  This prevents loading
    usigned images even when the kernel is not locked down at runtime.
    
    This patch splits KEXEC_VERIFY_SIG into KEXEC_SIG and KEXEC_SIG_FORCE.
    Analogous to the MODULE_SIG and MODULE_SIG_FORCE for modules, KEXEC_SIG
    turns on the signature verification but allows unsigned images to be
    loaded.  KEXEC_SIG_FORCE disallows images without a valid signature.
    
    [Modified by David Howells such that:
    
     (1) verify_pefile_signature() differentiates between no-signature and
         sig-didn't-match in its returned errors.
    
     (2) kexec fails with EKEYREJECTED and logs an appropriate message if
         signature checking is enforced and an signature is not found, uses
         unsupported crypto or has no matching key.
    
     (3) kexec fails with EKEYREJECTED if there is a signature for which we
         have a key, but signature doesn't match - even if in non-forcing mode.
    
     (4) kexec fails with EBADMSG or some other error if there is a signature
         which cannot be parsed - even if in non-forcing mode.
    
     (5) kexec fails with ELIBBAD if the PE file cannot be parsed to extract
         the signature - even if in non-forcing mode.
    
    ]
    
    Signed-off-by: Jiri Bohac <jbohac@xxxxxxx>
    Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
    cc: Matthew Garrett <mjg59@xxxxxxxxxxxxx>
    cc: Chun-Yi Lee <jlee@xxxxxxxx>
    cc: kexec@xxxxxxxxxxxxxxxxxxx

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index c1236b187824..cb6e67b7442d 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2019,20 +2019,30 @@ config KEXEC_FILE
 	  for kernel and initramfs as opposed to list of segments as
 	  accepted by previous system call.
 
-config KEXEC_VERIFY_SIG
+config KEXEC_SIG
 	bool "Verify kernel signature during kexec_file_load() syscall"
 	depends on KEXEC_FILE
 	---help---
-	  This option makes kernel signature verification mandatory for
-	  the kexec_file_load() syscall.
 
-	  In addition to that option, you need to enable signature
+	  This option makes the kexec_file_load() syscall check for a valid
+	  signature of the kernel image.  The image can still be loaded without
+	  a valid signature unless you also enable KEXEC_SIG_FORCE, though if
+	  there's a signature that we can check, then it must be valid.
+
+	  In addition to this option, you need to enable signature
 	  verification for the corresponding kernel image type being
 	  loaded in order for this to work.
 
+config KEXEC_SIG_FORCE
+	bool "Require a valid signature in kexec_file_load() syscall"
+	depends on KEXEC_SIG
+	---help---
+	  This option makes kernel signature verification mandatory for
+	  the kexec_file_load() syscall.
+
 config KEXEC_BZIMAGE_VERIFY_SIG
 	bool "Enable bzImage signature verification support"
-	depends on KEXEC_VERIFY_SIG
+	depends on KEXEC_SIG
 	depends on SIGNED_PE_FILE_VERIFICATION
 	select SYSTEM_TRUSTED_KEYRING
 	---help---
diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
index 1f790cf9d38f..3fbe35b923ef 100644
--- a/arch/x86/kernel/machine_kexec_64.c
+++ b/arch/x86/kernel/machine_kexec_64.c
@@ -406,7 +406,7 @@ int arch_kimage_file_post_load_cleanup(struct kimage *image)
 	return image->fops->cleanup(image->image_loader_data);
 }
 
-#ifdef CONFIG_KEXEC_VERIFY_SIG
+#ifdef CONFIG_KEXEC_SIG
 int arch_kexec_kernel_verify_sig(struct kimage *image, void *kernel,
 				 unsigned long kernel_len)
 {
diff --git a/crypto/asymmetric_keys/verify_pefile.c b/crypto/asymmetric_keys/verify_pefile.c
index d178650fd524..4473cea1e877 100644
--- a/crypto/asymmetric_keys/verify_pefile.c
+++ b/crypto/asymmetric_keys/verify_pefile.c
@@ -100,7 +100,7 @@ static int pefile_parse_binary(const void *pebuf, unsigned int pelen,
 
 	if (!ddir->certs.virtual_address || !ddir->certs.size) {
 		pr_debug("Unsigned PE binary\n");
-		return -EKEYREJECTED;
+		return -ENODATA;
 	}
 
 	chkaddr(ctx->header_size, ddir->certs.virtual_address,
@@ -408,6 +408,8 @@ static int pefile_digest_pe(const void *pebuf, unsigned int pelen,
  *  (*) 0 if at least one signature chain intersects with the keys in the trust
  *	keyring, or:
  *
+ *  (*) -ENODATA if there is no signature present.
+ *
  *  (*) -ENOPKG if a suitable crypto module couldn't be found for a check on a
  *	chain.
  *
diff --git a/include/linux/kexec.h b/include/linux/kexec.h
index f16f6ceb3875..19652372f3ee 100644
--- a/include/linux/kexec.h
+++ b/include/linux/kexec.h
@@ -121,7 +121,7 @@ typedef void *(kexec_load_t)(struct kimage *image, char *kernel_buf,
 			     unsigned long cmdline_len);
 typedef int (kexec_cleanup_t)(void *loader_data);
 
-#ifdef CONFIG_KEXEC_VERIFY_SIG
+#ifdef CONFIG_KEXEC_SIG
 typedef int (kexec_verify_sig_t)(const char *kernel_buf,
 				 unsigned long kernel_len);
 #endif
@@ -130,7 +130,7 @@ struct kexec_file_ops {
 	kexec_probe_t *probe;
 	kexec_load_t *load;
 	kexec_cleanup_t *cleanup;
-#ifdef CONFIG_KEXEC_VERIFY_SIG
+#ifdef CONFIG_KEXEC_SIG
 	kexec_verify_sig_t *verify_sig;
 #endif
 };
diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
index e5bcd94c1efb..d5931e392050 100644
--- a/kernel/kexec_file.c
+++ b/kernel/kexec_file.c
@@ -45,7 +45,7 @@ int __weak arch_kimage_file_post_load_cleanup(struct kimage *image)
 	return -EINVAL;
 }
 
-#ifdef CONFIG_KEXEC_VERIFY_SIG
+#ifdef CONFIG_KEXEC_SIG
 int __weak arch_kexec_kernel_verify_sig(struct kimage *image, void *buf,
 					unsigned long buf_len)
 {
@@ -116,7 +116,8 @@ kimage_file_prepare_segments(struct kimage *image, int kernel_fd, int initrd_fd,
 			     const char __user *cmdline_ptr,
 			     unsigned long cmdline_len, unsigned flags)
 {
-	int ret = 0;
+	const char *reason;
+	int ret;
 	void *ldata;
 	loff_t size;
 
@@ -135,15 +136,48 @@ kimage_file_prepare_segments(struct kimage *image, int kernel_fd, int initrd_fd,
 	if (ret)
 		goto out;
 
-#ifdef CONFIG_KEXEC_VERIFY_SIG
+#ifdef CONFIG_KEXEC_SIG
 	ret = arch_kexec_kernel_verify_sig(image, image->kernel_buf,
 					   image->kernel_buf_len);
-	if (ret) {
-		pr_debug("kernel signature verification failed.\n");
+#else
+	ret = -ENODATA;
+#endif
+
+	switch (ret) {
+	case 0:
+		break;
+
+		/* Certain verification errors are non-fatal if we're not
+		 * checking errors, provided we aren't mandating that there
+		 * must be a valid signature.
+		 */
+	case -ENODATA:
+		reason = "kexec of unsigned image";
+		goto decide;
+	case -ENOPKG:
+		reason = "kexec of image with unsupported crypto";
+		goto decide;
+	case -ENOKEY:
+		reason = "kexec of image with unavailable key";
+	decide:
+		if (IS_ENABLED(CONFIG_KEXEC_SIG_FORCE)) {
+			pr_notice("%s rejected\n", reason);
+			ret = -EKEYREJECTED;
+			goto out;
+		}
+
+		ret = 0;
+		break;
+
+		/* All other errors are fatal, including nomem, unparseable
+		 * signatures and signature check failures - even if signatures
+		 * aren't required.
+		 */
+	default:
+		pr_notice("kernel signature verification failed (%d).\n", ret);
 		goto out;
 	}
-	pr_debug("kernel signature verification successful.\n");
-#endif
+
 	/* It is possible that there no initramfs is being loaded */
 	if (!(flags & KEXEC_FILE_NO_INITRAMFS)) {
 		ret = kernel_read_file_from_fd(initrd_fd, &image->initrd_buf,