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@xxxxxxxxxx>
---
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
