evm: Enforce signatures version 3 with new EVM policy 'bit 3'

Enable the configuration of EVM so that it requires that asymmetric
signatures it accepts are of version 3 (sigv3). To enable this, introduce
bit 3 (value 0x0008) that the user may write to EVM's securityfs policy
configuration file 'evm' for sigv3 enforcement.

Mention bit 3 in the documentation.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

Author: stefanberger

Documentation/ABI/testing/evm

@@ -26,6 +26,7 @@ Description:
 		2	  Permit modification of EVM-protected metadata at
 			  runtime. Not supported if HMAC validation and
 			  creation is enabled (deprecated).
+		3	  Require asymmetric signatures to be version 3
 		31	  Disable further runtime modification of EVM policy
 		===	  ==================================================
 

security/integrity/evm/evm.h

@@ -20,11 +20,12 @@
 #define EVM_INIT_HMAC	0x0001
 #define EVM_INIT_X509	0x0002
 #define EVM_ALLOW_METADATA_WRITES	0x0004
+#define EVM_SIGV3_REQUIRED		0x0008
 #define EVM_SETUP_COMPLETE 0x80000000 /* userland has signaled key load */
 
 #define EVM_KEY_MASK (EVM_INIT_HMAC | EVM_INIT_X509)
 #define EVM_INIT_MASK (EVM_INIT_HMAC | EVM_INIT_X509 | EVM_SETUP_COMPLETE | \
-		       EVM_ALLOW_METADATA_WRITES)
+		       EVM_ALLOW_METADATA_WRITES | EVM_SIGV3_REQUIRED)
 
 struct xattr_list {
 	struct list_head list;

security/integrity/evm/evm_main.c

@@ -136,6 +136,14 @@ static bool evm_hmac_disabled(void)
 	return true;
 }
 
+static bool evm_sigv3_required(void)
+{
+	if (evm_initialized & EVM_SIGV3_REQUIRED)
+		return true;
+
+	return false;
+}
+
 static int evm_find_protected_xattrs(struct dentry *dentry)
 {
 	struct inode *inode = d_backing_inode(dentry);
@@ -258,6 +266,12 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
 		}
 
 		hdr = (struct signature_v2_hdr *)xattr_data;
+
+		if (evm_sigv3_required() && hdr->version != 3) {
+			evm_status = INTEGRITY_FAIL;
+			goto out;
+		}
+
 		digest.hdr.algo = hdr->hash_algo;
 		rc = evm_calc_hash(dentry, xattr_name, xattr_value,
 				   xattr_value_len, xattr_data->type, &digest,