[PATCH v3 06/11] evm: Ignore INTEGRITY_NOLABEL if no HMAC key is loaded

[Roberto Sassu]

When a file is being created, LSMs can set the initial label with the
inode_init_security hook. If no HMAC key is loaded, the new file will have
LSM xattrs but not the HMAC.

Unfortunately, EVM will deny any further metadata operation on new files,
as evm_protect_xattr() will always return the INTEGRITY_NOLABEL error. This
would limit the usability of EVM when only a public key is loaded, as
commands such as cp or tar with the option to preserve xattrs won't work.

Ignoring this error won't be an issue if no HMAC key is loaded, as the
inode is locked until the post hook, and EVM won't calculate the HMAC on
metadata that wasn't previously verified. Thus this patch checks if an
HMAC key is loaded and if not, ignores INTEGRITY_NOLABEL.

Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
---
 security/integrity/evm/evm_main.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index b38ffa39faa8..4f4404a12bbd 100644
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -354,6 +354,14 @@ static int evm_protect_xattr(struct dentry *dentry, const char *xattr_name,
 				    -EPERM, 0);
 	}
 out:
+	/*
+	 * Ignoring INTEGRITY_NOLABEL is safe if no HMAC key is loaded, as
+	 * EVM won't calculate the HMAC of metadata that wasn't previously
+	 * verified.
+	 */
+	if (evm_status == INTEGRITY_NOLABEL &&
+	    !(evm_initialized & EVM_INIT_HMAC))
+		return 0;
 	if (evm_status != INTEGRITY_PASS)
 		integrity_audit_msg(AUDIT_INTEGRITY_METADATA, d_backing_inode(dentry),
 				    dentry->d_name.name, "appraise_metadata",
@@ -514,8 +522,15 @@ int evm_inode_setattr(struct dentry *dentry, struct iattr *attr)
 	if (!(ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID)))
 		return 0;
 	evm_status = evm_verify_current_integrity(dentry);
+	/*
+	 * Ignoring INTEGRITY_NOLABEL is safe if no HMAC key is loaded, as
+	 * EVM won't calculate the HMAC of metadata that wasn't previously
+	 * verified.
+	 */
 	if ((evm_status == INTEGRITY_PASS) ||
-	    (evm_status == INTEGRITY_NOXATTRS))
+	    (evm_status == INTEGRITY_NOXATTRS) ||
+	    (evm_status == INTEGRITY_NOLABEL &&
+	     !(evm_initialized & EVM_INIT_HMAC)))
 		return 0;
 	integrity_audit_msg(AUDIT_INTEGRITY_METADATA, d_backing_inode(dentry),
 			    dentry->d_name.name, "appraise_metadata",
-- 
2.27.GIT