[PATCH 27/27] kexec: Allow kexec_file() with appropriate IMA policy when locked down

[Matthew Garrett]

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 has or 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@xxxxxxxxxx>
Cc: Mimi Zohar <zohar@xxxxxxxxxxxxx>
Cc: Dmitry Kasatkin <dmitry.kasatkin@xxxxxxxxx>
Cc: linux-integrity@xxxxxxxxxxxxxxx
---
 include/linux/ima.h                 |  9 ++++++
 kernel/kexec_file.c                 |  7 +++-
 security/integrity/ima/ima.h        |  2 ++
 security/integrity/ima/ima_main.c   |  2 +-
 security/integrity/ima/ima_policy.c | 50 +++++++++++++++++++++++++++++
 5 files changed, 68 insertions(+), 2 deletions(-)

diff --git a/include/linux/ima.h b/include/linux/ima.h
index b5e16b8c50b7..05921227d700 100644
--- a/include/linux/ima.h
+++ b/include/linux/ima.h
@@ -127,4 +127,13 @@ 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 kernel_read_file_id func);
+#else
+static inline bool ima_appraise_kexec_signature(enum kernel_read_file_id 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..8ffa4b75c620 100644
--- a/kernel/kexec_file.c
+++ b/kernel/kexec_file.c
@@ -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(READING_KEXEC_IMAGE) &&
+		    kernel_is_locked_down(reason)) {
 			ret = -EPERM;
 			goto out;
 		}
diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
index cc12f3449a72..fe03cc6f1ca4 100644
--- a/security/integrity/ima/ima.h
+++ b/security/integrity/ima/ima.h
@@ -115,6 +115,8 @@ struct ima_kexec_hdr {
 	u64 count;
 };
 
+extern const int read_idmap[];
+
 #ifdef CONFIG_HAVE_IMA_KEXEC
 void ima_load_kexec_buffer(void);
 #else
diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
index 4ffac4f5c647..106f06dee9d1 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -442,7 +442,7 @@ int ima_read_file(struct file *file, enum kernel_read_file_id read_id)
 	return 0;
 }
 
-static const int read_idmap[READING_MAX_ID] = {
+const int read_idmap[READING_MAX_ID] = {
 	[READING_FIRMWARE] = FIRMWARE_CHECK,
 	[READING_FIRMWARE_PREALLOC_BUFFER] = FIRMWARE_CHECK,
 	[READING_MODULE] = MODULE_CHECK,
diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
index 122797023bdb..f8f1cdb74a4f 100644
--- a/security/integrity/ima/ima_policy.c
+++ b/security/integrity/ima/ima_policy.c
@@ -1341,3 +1341,53 @@ int ima_policy_show(struct seq_file *m, void *v)
 	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 kernel_read_file_id id)
+{
+	struct ima_rule_entry *entry;
+	bool found = false;
+	enum ima_hooks func;
+
+	if (id >= READING_MAX_ID)
+		return false;
+
+	func = read_idmap[id] ?: FILE_CHECK;
+
+	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 */
-- 
2.21.0.392.gf8f6787159e-goog