[PATCH v7 03/12] evm: Refuse EVM_ALLOW_METADATA_WRITES only if an HMAC key is loaded

[Roberto Sassu]

EVM_ALLOW_METADATA_WRITES is an EVM initialization flag that can be set to
temporarily disable metadata verification until all xattrs/attrs necessary
to verify an EVM portable signature are copied to the file. This flag is
cleared when EVM is initialized with an HMAC key, to avoid that the HMAC is
calculated on unverified xattrs/attrs.

Currently EVM unnecessarily denies setting this flag if EVM is initialized
with a public key, which is not a concern as it cannot be used to trust
xattrs/attrs updates. This patch removes this limitation.

Cc: stable at vger.kernel.org # 4.16.x
Fixes: ae1ba1676b88e ("EVM: Allow userland to permit modification of EVM-protected metadata")
Signed-off-by: Roberto Sassu <roberto.sassu at huawei.com>
Reviewed-by: Mimi Zohar <zohar at linux.ibm.com>
---
 Documentation/ABI/testing/evm      | 26 ++++++++++++++++++++++++--
 security/integrity/evm/evm_secfs.c |  8 ++++----
 2 files changed, 28 insertions(+), 6 deletions(-)

diff --git a/Documentation/ABI/testing/evm b/Documentation/ABI/testing/evm
index 3c477ba48a31..2243b72e4110 100644
--- a/Documentation/ABI/testing/evm
+++ b/Documentation/ABI/testing/evm
@@ -49,8 +49,30 @@ Description:
 		modification of EVM-protected metadata and
 		disable all further modification of policy
 
-		Note that once a key has been loaded, it will no longer be
-		possible to enable metadata modification.
+		Echoing a value is additive, the new value is added to the
+		existing initialization flags.
+
+		For example, after::
+
+		  echo 2 ><securityfs>/evm
+
+		another echo can be performed::
+
+		  echo 1 ><securityfs>/evm
+
+		and the resulting value will be 3.
+
+		Note that once an HMAC key has been loaded, it will no longer
+		be possible to enable metadata modification. Signaling that an
+		HMAC key has been loaded will clear the corresponding flag.
+		For example, if the current value is 6 (2 and 4 set)::
+
+		  echo 1 ><securityfs>/evm
+
+		will set the new value to 3 (4 cleared).
+
+		Loading an HMAC key is the only way to disable metadata
+		modification.
 
 		Until key loading has been signaled EVM can not create
 		or validate the 'security.evm' xattr, but returns
diff --git a/security/integrity/evm/evm_secfs.c b/security/integrity/evm/evm_secfs.c
index bbc85637e18b..c175e2b659e4 100644
--- a/security/integrity/evm/evm_secfs.c
+++ b/security/integrity/evm/evm_secfs.c
@@ -80,12 +80,12 @@ static ssize_t evm_write_key(struct file *file, const char __user *buf,
 	if (!i || (i & ~EVM_INIT_MASK) != 0)
 		return -EINVAL;
 
-	/* Don't allow a request to freshly enable metadata writes if
-	 * keys are loaded.
+	/*
+	 * Don't allow a request to enable metadata writes if
+	 * an HMAC key is loaded.
 	 */
 	if ((i & EVM_ALLOW_METADATA_WRITES) &&
-	    ((evm_initialized & EVM_KEY_MASK) != 0) &&
-	    !(evm_initialized & EVM_ALLOW_METADATA_WRITES))
+	    (evm_initialized & EVM_INIT_HMAC) != 0)
 		return -EPERM;
 
 	if (i & EVM_INIT_HMAC) {
-- 
2.25.1