evm: Enforce signatures on unsupported filesystem for EVM_INIT_X509

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.

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

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 without EVM_INIT_X509 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 */
@@ -261,7 +265,8 @@ 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);
@@ -419,9 +424,6 @@ 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;
-
  return evm_verify_hmac(dentry, xattr_name, xattr_value,
  xattr_value_len);
 }