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 instead, either with native IMA digital
signatures or EVM-protected IMA hashes. Add a function to determine
whether IMA will verify signatures on kexec files, 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 | 9 ++++++++
kernel/kexec_file.c | 9 ++++++--
security/integrity/evm/evm_main.c | 2 +-
security/integrity/ima/ima_policy.c | 35 +++++++++++++++++++++++++++++
5 files changed, 58 insertions(+), 3 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..2ec593537c9b 100644
--- a/include/linux/ima.h
+++ b/include/linux/ima.h
@@ -132,4 +132,13 @@ 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_kexec_signature(void);
+#else
+static inline bool ima_appraise_kexec_signature(void)
+{
+ 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..8ca607f1b515 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_kexec_signature() &&
+ 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_policy.c b/security/integrity/ima/ima_policy.c
index 8bc8a1c8cb3f..c06b1a6b3528 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 +1337,38 @@ 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_kexec: whether IMA will appraise a kexec image, either via
+ * IMA digital signatures or with a hash and EVM validation
+ */
+bool ima_appraise_kexec_signature(void)
+{
+ struct ima_rule_entry *entry;
+ bool found = false;
+
+ rcu_read_lock();
+ list_for_each_entry_rcu(entry, ima_rules, list) {
+ if (entry->func != KEXEC_KERNEL_CHECK ||
+ entry->action != APPRAISE)
+ continue;
+
+ /*
+ * We require this to be a digital signature, not a raw IMA
+ * hash. An IMA hash is acceptable as long as it's covered
+ * by an EVM signature.
+ */
+ if (entry->flags & IMA_DIGSIG_REQUIRED ||
+ evm_key_loaded()) {
+ found = true;
+ break;
+ }
+ }
+
+ rcu_read_unlock();
+ return found;
+}
+#endif /* CONFIG_IMA_APPRAISE && CONFIG_INTEGRITY_TRUSTED_KEYRING */
--
2.21.0.360.g471c308f928-goog
