[PATCH v6 11/12] ima: Implement support for module-style appended signatures

[Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com>]

This patch actually implements the appraise_type=imasig|modsig option,
allowing IMA to read and verify modsig signatures.

In case both are present in the same file, IMA will first check whether the
key used by the xattr signature is present in the kernel keyring. If not,
it will try the appended signature.

Signed-off-by: Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com>
---
 security/integrity/ima/ima.h          | 11 +++++++-
 security/integrity/ima/ima_appraise.c | 53 +++++++++++++++++++++++++++++++----
 security/integrity/ima/ima_main.c     | 21 +++++++++++---
 3 files changed, 74 insertions(+), 11 deletions(-)

diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
index 49aef56dc96d..c11ccb7c5bfb 100644
--- a/security/integrity/ima/ima.h
+++ b/security/integrity/ima/ima.h
@@ -157,7 +157,8 @@ void ima_init_template_list(void);
 
 static inline bool is_ima_sig(const struct evm_ima_xattr_data *xattr_value)
 {
-	return xattr_value && xattr_value->type == EVM_IMA_XATTR_DIGSIG;
+	return xattr_value && (xattr_value->type == EVM_IMA_XATTR_DIGSIG ||
+			       xattr_value->type == IMA_MODSIG);
 }
 
 /*
@@ -253,6 +254,8 @@ enum integrity_status ima_get_cache_status(struct integrity_iint_cache *iint,
 					   enum ima_hooks func);
 enum hash_algo ima_get_hash_algo(struct evm_ima_xattr_data *xattr_value,
 				 int xattr_len);
+bool ima_xattr_sig_known_key(const struct evm_ima_xattr_data *xattr_value,
+			     int xattr_len);
 int ima_read_xattr(struct dentry *dentry,
 		   struct evm_ima_xattr_data **xattr_value);
 
@@ -291,6 +294,12 @@ ima_get_hash_algo(struct evm_ima_xattr_data *xattr_value, int xattr_len)
 	return ima_hash_algo;
 }
 
+static inline bool ima_xattr_sig_known_key(const struct evm_ima_xattr_data
+					   *xattr_value, int xattr_len)
+{
+	return false;
+}
+
 static inline int ima_read_xattr(struct dentry *dentry,
 				 struct evm_ima_xattr_data **xattr_value)
 {
diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
index 01172eab297b..84e0fd5a19c8 100644
--- a/security/integrity/ima/ima_appraise.c
+++ b/security/integrity/ima/ima_appraise.c
@@ -189,6 +189,22 @@ enum hash_algo ima_get_hash_algo(struct evm_ima_xattr_data *xattr_value,
 	return ima_hash_algo;
 }
 
+bool ima_xattr_sig_known_key(const struct evm_ima_xattr_data *xattr_value,
+			     int xattr_len)
+{
+	struct key *keyring;
+
+	if (xattr_value->type != EVM_IMA_XATTR_DIGSIG)
+		return false;
+
+	keyring = integrity_keyring_from_id(INTEGRITY_KEYRING_IMA);
+	if (IS_ERR(keyring))
+		return false;
+
+	return asymmetric_sig_has_known_key(keyring, (const char *) xattr_value,
+					    xattr_len);
+}
+
 int ima_read_xattr(struct dentry *dentry,
 		   struct evm_ima_xattr_data **xattr_value)
 {
@@ -221,8 +237,12 @@ int ima_appraise_measurement(enum ima_hooks func,
 	struct inode *inode = d_backing_inode(dentry);
 	enum integrity_status status = INTEGRITY_UNKNOWN;
 	int rc = xattr_len, hash_start = 0;
+	size_t xattr_contents_len;
+	void *xattr_contents;
 
-	if (!(inode->i_opflags & IOP_XATTR))
+	/* If not appraising a modsig, we need an xattr. */
+	if ((xattr_value == NULL || xattr_value->type != IMA_MODSIG) &&
+	    !(inode->i_opflags & IOP_XATTR))
 		return INTEGRITY_UNKNOWN;
 
 	if (rc <= 0) {
@@ -241,13 +261,29 @@ int ima_appraise_measurement(enum ima_hooks func,
 		goto out;
 	}
 
-	status = evm_verifyxattr(dentry, XATTR_NAME_IMA, xattr_value, rc, iint);
+	/*
+	 * If it's a modsig, we don't have the xattr contents to pass to
+	 * evm_verifyxattr().
+	 */
+	if (xattr_value->type == IMA_MODSIG) {
+		xattr_contents = NULL;
+		xattr_contents_len = 0;
+	} else {
+		xattr_contents = xattr_value;
+		xattr_contents_len = xattr_len;
+	}
+
+	status = evm_verifyxattr(dentry, XATTR_NAME_IMA, xattr_contents,
+				 xattr_contents_len, iint);
 	switch (status) {
 	case INTEGRITY_PASS:
 	case INTEGRITY_PASS_IMMUTABLE:
 	case INTEGRITY_UNKNOWN:
 		break;
 	case INTEGRITY_NOXATTRS:	/* No EVM protected xattrs. */
+		/* It's fine not to have xattrs when using a modsig. */
+		if (xattr_value->type == IMA_MODSIG)
+			break;
 	case INTEGRITY_NOLABEL:		/* No security.evm xattr. */
 		cause = "missing-HMAC";
 		goto out;
@@ -288,11 +324,16 @@ int ima_appraise_measurement(enum ima_hooks func,
 		status = INTEGRITY_PASS;
 		break;
 	case EVM_IMA_XATTR_DIGSIG:
+	case IMA_MODSIG:
 		set_bit(IMA_DIGSIG, &iint->atomic_flags);
-		rc = integrity_digsig_verify(INTEGRITY_KEYRING_IMA,
-					     (const char *)xattr_value, rc,
-					     iint->ima_hash->digest,
-					     iint->ima_hash->length);
+		if (xattr_value->type == EVM_IMA_XATTR_DIGSIG)
+			rc = integrity_digsig_verify(INTEGRITY_KEYRING_IMA,
+						     (const char *)xattr_value,
+						     rc, iint->ima_hash->digest,
+						     iint->ima_hash->length);
+		else
+			rc = ima_modsig_verify(INTEGRITY_KEYRING_IMA,
+					       xattr_value);
 		if (rc == -EOPNOTSUPP) {
 			status = INTEGRITY_UNKNOWN;
 		} else if (rc) {
diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
index 5d122daf5c8a..1b11c10f09df 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -183,7 +183,7 @@ static int process_measurement(struct file *file, const struct cred *cred,
 	struct evm_ima_xattr_data *xattr_value = NULL;
 	int xattr_len = 0;
 	bool violation_check;
-	enum hash_algo hash_algo;
+	enum hash_algo hash_algo = HASH_ALGO__LAST;
 
 	if (!ima_policy_flag || !S_ISREG(inode->i_mode))
 		return 0;
@@ -277,11 +277,24 @@ static int process_measurement(struct file *file, const struct cred *cred,
 
 	template_desc = ima_template_desc_current();
 	if ((action & IMA_APPRAISE_SUBMASK) ||
-		    strcmp(template_desc->name, IMA_TEMPLATE_IMA_NAME) != 0)
+	    strcmp(template_desc->name, IMA_TEMPLATE_IMA_NAME) != 0) {
 		/* read 'security.ima' */
 		xattr_len = ima_read_xattr(file_dentry(file), &xattr_value);
+		if (iint->flags & IMA_MODSIG_ALLOWED &&
+		    (xattr_len <= 0 || !ima_xattr_sig_known_key(xattr_value,
+								xattr_len))) {
+			/*
+			 * Even if we end up using a modsig, hash_algo should
+			 * come from the xattr (or even the default hash algo).
+			 */
+			hash_algo = ima_get_hash_algo(xattr_value, xattr_len);
+			ima_read_modsig(func, buf, size, &xattr_value,
+					&xattr_len);
+		}
+	}
 
-	hash_algo = ima_get_hash_algo(xattr_value, xattr_len);
+	if (hash_algo == HASH_ALGO__LAST)
+		hash_algo = ima_get_hash_algo(xattr_value, xattr_len);
 
 	rc = ima_collect_measurement(iint, file, buf, size, hash_algo);
 	if (rc != 0 && rc != -EBADF && rc != -EINVAL)
@@ -309,7 +322,7 @@ static int process_measurement(struct file *file, const struct cred *cred,
 	     !(iint->flags & IMA_NEW_FILE))
 		rc = -EACCES;
 	mutex_unlock(&iint->mutex);
-	kfree(xattr_value);
+	ima_free_xattr_data(xattr_value);
 out:
 	if (pathbuf)
 		__putname(pathbuf);