[PATCH 5/5] evm: Enforce signatures on unsupported filesystem for EVM_INIT_X509

[Stefan Berger]

Unsupported filesystems currently do not enforce any signatures. Add
support for signature enforcement of the "original" and "portable &
immutable" signatures when EVM_INIT_X509 is enabled.

The "original" signature type contains filesystem specific metadata.
Thus it cannot be copied up and verified. However with EVM_INIT_X509
and EVM_ALLOW_METADATA_WRITES enabled, the "original" file signature
may be written.

When EVM_ALLOW_METADATA_WRITES is not set or once it is removed from
/sys/kernel/security/evm by setting EVM_INIT_HMAC for example, it is not
possible to write or remove xattrs on the overlay filesystem.

This change still prevents EVM from writing HMAC signatures on
unsupported filesystem when EVM_INIT_HMAC is enabled.

Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
---
 security/integrity/evm/evm_main.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index e96d127b48a2..f49609dfcbc7 100644
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -192,7 +192,11 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
 		     iint->evm_status == INTEGRITY_PASS_IMMUTABLE))
 		return iint->evm_status;
 
-	if (is_unsupported_fs(dentry))
+	/*
+	 * On unsupported filesystems with EVM_INIT_X509 not enabled, skip
+	 * signature verification.
+	 */
+	if (!(evm_initialized & EVM_INIT_X509) && is_unsupported_fs(dentry))
 		return INTEGRITY_UNKNOWN;
 
 	/* if status is not PASS, try to check again - against -ENOMEM */
@@ -262,7 +266,8 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
 				evm_status = INTEGRITY_PASS_IMMUTABLE;
 			} else if (!IS_RDONLY(inode) &&
 				   !(inode->i_sb->s_readonly_remount) &&
-				   !IS_IMMUTABLE(inode)) {
+				   !IS_IMMUTABLE(inode) &&
+				   !is_unsupported_fs(dentry)) {
 				evm_update_evmxattr(dentry, xattr_name,
 						    xattr_value,
 						    xattr_value_len);
@@ -422,9 +427,6 @@ enum integrity_status evm_verifyxattr(struct dentry *dentry,
 	if (!evm_key_loaded() || !evm_protected_xattr(xattr_name))
 		return INTEGRITY_UNKNOWN;
 
-	if (is_unsupported_fs(dentry))
-		return INTEGRITY_UNKNOWN;
-
 	if (!iint) {
 		iint = integrity_iint_find(d_backing_inode(dentry));
 		if (!iint)
-- 
2.43.0