[RFC] kexec: Allow kexec_file() with appropriate IMA policy when locked down

[Matthew Garrett <matthewgarrett@google.com>]

Systems in lockdown mode should block the kexec of untrusted kernels.
For x86 and ARM we can ensure that a kernel is trustworthy by validating
a PE signature, but this isn't possible on other architectures. On those
platforms we can use IMA digital signatures instead. Add a function to
determine whether IMA will verify signatures for a given event type, and
if so permit kexec_file() even if the kernel is otherwise locked down.
This is restricted to cases where CONFIG_INTEGRITY_TRUSTED_KEYRING is set
in order to prevent an attacker from loading additional keys at runtime.

Signed-off-by: Matthew Garrett <mjg59@google.com>
---
 include/linux/evm.h                 |  6 ++++
 include/linux/ima.h                 | 28 +++++++++++++++++++
 kernel/kexec_file.c                 |  9 ++++--
 security/integrity/evm/evm_main.c   |  2 +-
 security/integrity/ima/ima.h        | 20 +-------------
 security/integrity/ima/ima_policy.c | 43 +++++++++++++++++++++++++++++
 6 files changed, 86 insertions(+), 22 deletions(-)

diff --git a/include/linux/evm.h b/include/linux/evm.h
index 8302bc29bb35..6e89d046b716 100644
--- a/include/linux/evm.h
+++ b/include/linux/evm.h
@@ -15,6 +15,7 @@
 struct integrity_iint_cache;
 
 #ifdef CONFIG_EVM
+extern bool evm_key_loaded(void);
 extern int evm_set_key(void *key, size_t keylen);
 extern enum integrity_status evm_verifyxattr(struct dentry *dentry,
 					     const char *xattr_name,
@@ -45,6 +46,11 @@ static inline int posix_xattr_acl(const char *xattrname)
 #endif
 #else
 
+static inline bool evm_key_loaded(void)
+{
+	return false;
+}
+
 static inline int evm_set_key(void *key, size_t keylen)
 {
 	return -EOPNOTSUPP;
diff --git a/include/linux/ima.h b/include/linux/ima.h
index dc12fbcf484c..a42e2a9a08b7 100644
--- a/include/linux/ima.h
+++ b/include/linux/ima.h
@@ -27,6 +27,25 @@ extern int ima_post_read_file(struct file *file, void *buf, loff_t size,
 			      enum kernel_read_file_id id);
 extern void ima_post_path_mknod(struct dentry *dentry);
 
+#define __ima_hooks(hook)		\
+	hook(NONE)			\
+	hook(FILE_CHECK)		\
+	hook(MMAP_CHECK)		\
+	hook(BPRM_CHECK)		\
+	hook(CREDS_CHECK)		\
+	hook(POST_SETATTR)		\
+	hook(MODULE_CHECK)		\
+	hook(FIRMWARE_CHECK)		\
+	hook(KEXEC_KERNEL_CHECK)	\
+	hook(KEXEC_INITRAMFS_CHECK)	\
+	hook(POLICY_CHECK)		\
+	hook(MAX_CHECK)
+#define __ima_hook_enumify(ENUM)	ENUM,
+
+enum ima_hooks {
+	__ima_hooks(__ima_hook_enumify)
+};
+
 #ifdef CONFIG_IMA_KEXEC
 extern void ima_add_kexec_buffer(struct kimage *image);
 #endif
@@ -132,4 +151,13 @@ static inline int ima_inode_removexattr(struct dentry *dentry,
 	return 0;
 }
 #endif /* CONFIG_IMA_APPRAISE */
+
+#if defined(CONFIG_IMA_APPRAISE) && defined(CONFIG_INTEGRITY_TRUSTED_KEYRING)
+extern bool ima_appraise_signature(enum ima_hooks func);
+#else
+static inline bool ima_appraise_kexec_signature(enum ima_hooks func)
+{
+	return false;
+}
+#endif /* CONFIG_IMA_APPRAISE && CONFIG_INTEGRITY_TRUSTED_KEYRING */
 #endif /* _LINUX_IMA_H */
diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
index 0cfe4f6f7f85..3e04506a00a2 100644
--- a/kernel/kexec_file.c
+++ b/kernel/kexec_file.c
@@ -20,11 +20,11 @@
 #include <linux/mutex.h>
 #include <linux/list.h>
 #include <linux/fs.h>
-#include <linux/ima.h>
 #include <crypto/hash.h>
 #include <crypto/sha.h>
 #include <linux/elf.h>
 #include <linux/elfcore.h>
+#include <linux/ima.h>
 #include <linux/kernel.h>
 #include <linux/syscalls.h>
 #include <linux/vmalloc.h>
@@ -240,7 +240,12 @@ kimage_file_prepare_segments(struct kimage *image, int kernel_fd, int initrd_fd,
 
 		ret = 0;
 
-		if (kernel_is_locked_down(reason)) {
+		/* If IMA is guaranteed to appraise a signature on the kexec
+		 * image, permit it even if the kernel is otherwise locked
+		 * down.
+		 */
+		if (!ima_appraise_signature(KEXEC_KERNEL_CHECK) &&
+		    kernel_is_locked_down(reason)) {
 			ret = -EPERM;
 			goto out;
 		}
diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index b6d9f14bc234..aad61bc0f774 100644
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -87,7 +87,7 @@ static void __init evm_init_config(void)
 	pr_info("HMAC attrs: 0x%x\n", evm_hmac_attrs);
 }
 
-static bool evm_key_loaded(void)
+bool evm_key_loaded(void)
 {
 	return (bool)(evm_initialized & EVM_KEY_MASK);
 }
diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
index cc12f3449a72..71614a8ed2aa 100644
--- a/security/integrity/ima/ima.h
+++ b/security/integrity/ima/ima.h
@@ -20,6 +20,7 @@
 #include <linux/types.h>
 #include <linux/crypto.h>
 #include <linux/fs.h>
+#include <linux/ima.h>
 #include <linux/security.h>
 #include <linux/hash.h>
 #include <linux/tpm.h>
@@ -171,25 +172,6 @@ static inline unsigned long ima_hash_key(u8 *digest)
 	return hash_long(*digest, IMA_HASH_BITS);
 }
 
-#define __ima_hooks(hook)		\
-	hook(NONE)			\
-	hook(FILE_CHECK)		\
-	hook(MMAP_CHECK)		\
-	hook(BPRM_CHECK)		\
-	hook(CREDS_CHECK)		\
-	hook(POST_SETATTR)		\
-	hook(MODULE_CHECK)		\
-	hook(FIRMWARE_CHECK)		\
-	hook(KEXEC_KERNEL_CHECK)	\
-	hook(KEXEC_INITRAMFS_CHECK)	\
-	hook(POLICY_CHECK)		\
-	hook(MAX_CHECK)
-#define __ima_hook_enumify(ENUM)	ENUM,
-
-enum ima_hooks {
-	__ima_hooks(__ima_hook_enumify)
-};
-
 /* LIM API function definitions */
 int ima_get_action(struct inode *inode, const struct cred *cred, u32 secid,
 		   int mask, enum ima_hooks func, int *pcr);
diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
index 8bc8a1c8cb3f..adeae1ab9ee9 100644
--- a/security/integrity/ima/ima_policy.c
+++ b/security/integrity/ima/ima_policy.c
@@ -21,6 +21,7 @@
 #include <linux/genhd.h>
 #include <linux/seq_file.h>
 #include <linux/ima.h>
+#include <linux/evm.h>
 
 #include "ima.h"
 
@@ -1336,4 +1337,46 @@ int ima_policy_show(struct seq_file *m, void *v)
 	seq_puts(m, "\n");
 	return 0;
 }
+
 #endif	/* CONFIG_IMA_READ_POLICY */
+
+#if defined(CONFIG_IMA_APPRAISE) && defined(CONFIG_INTEGRITY_TRUSTED_KEYRING)
+/*
+ * ima_appraise_signature: whether IMA will appraise a given function using
+ * an IMA digital signature. This is restricted to cases where the kernel
+ * has a set of built-in trusted keys in order to avoid an attacker simply
+ * loading additional keys.
+ */
+bool ima_appraise_signature(enum ima_hooks func)
+{
+	struct ima_rule_entry *entry;
+	bool found = false;
+
+	rcu_read_lock();
+	list_for_each_entry_rcu(entry, ima_rules, list) {
+		if (entry->action != APPRAISE)
+			continue;
+
+		/* A generic entry will match, but otherwise require that it
+		 * match the func we're looking for
+		 */
+		if (entry->func && entry->func != func)
+			continue;
+
+		/* We require this to be a digital signature, not a raw IMA
+		 * hash.
+		 */
+		if (entry->flags & IMA_DIGSIG_REQUIRED)
+			found = true;
+
+		/* We've found a rule that matches, so break now even if it
+		 * didn't require a digital signature - a later rule that does
+		 * won't override it, so would be a false positive.
+		 */
+		break;
+	}
+
+	rcu_read_unlock();
+	return found;
+}
+#endif /* CONFIG_IMA_APPRAISE && CONFIG_INTEGRITY_TRUSTED_KEYRING */
-- 
2.21.0.225.g810b269d1ac-goog