EVM: Allow userspace to signal an RSA key has been loaded

EVM will only perform validation once a key has been loaded. This key
may either be a symmetric trusted key (for HMAC validation and creation)
or the public half of an asymmetric key (for digital signature
validation). The /sys/kernel/security/evm interface allows userland to
signal that a symmetric key has been loaded, but does not allow userland
to signal that an asymmetric public key has been loaded.

This patch extends the interface to permit userspace to pass a bitmask
of loaded key types. It also allows userspace to block loading of a
symmetric key in order to avoid a compromised system from being able to
load an additional key type later.

Signed-off-by: Matthew Garrett <mjg59@google.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>

Documentation/ABI/testing/evm

@@ -7,17 +7,36 @@ Description:
 		HMAC-sha1 value across the extended attributes, storing the
 		value as the extended attribute 'security.evm'.
 
-		EVM depends on the Kernel Key Retention System to provide it
-		with a trusted/encrypted key for the HMAC-sha1 operation.
-		The key is loaded onto the root's keyring using keyctl.  Until
-		EVM receives notification that the key has been successfully
-		loaded onto the keyring (echo 1 > <securityfs>/evm), EVM
-		can not create or validate the 'security.evm' xattr, but
-		returns INTEGRITY_UNKNOWN.  Loading the key and signaling EVM
-		should be done as early as possible.  Normally this is done
-		in the initramfs, which has already been measured as part
-		of the trusted boot.  For more information on creating and
-		loading existing trusted/encrypted keys, refer to:
-		Documentation/keys-trusted-encrypted.txt.  (A sample dracut
-		patch, which loads the trusted/encrypted key and enables
-		EVM, is available from http://linux-ima.sourceforge.net/#EVM.)
+		EVM supports two classes of security.evm. The first is
+		an HMAC-sha1 generated locally with a
+		trusted/encrypted key stored in the Kernel Key
+		Retention System. The second is a digital signature
+		generated either locally or remotely using an
+		asymmetric key. These keys are loaded onto root's
+		keyring using keyctl, and EVM is then enabled by
+		echoing a value to <securityfs>/evm:
+
+		1: enable HMAC validation and creation
+		2: enable digital signature validation
+		3: enable HMAC and digital signature validation and HMAC
+		   creation
+
+		Further writes will be blocked if HMAC support is enabled or
+		if bit 32 is set:
+
+		echo 0x80000002 ><securityfs>/evm
+
+		will enable digital signature validation and block
+		further writes to <securityfs>/evm.
+
+		Until this is done, EVM can not create or validate the
+		'security.evm' xattr, but returns INTEGRITY_UNKNOWN.
+		Loading keys and signaling EVM should be done as early
+		as possible.  Normally this is done in the initramfs,
+		which has already been measured as part of the trusted
+		boot.  For more information on creating and loading
+		existing trusted/encrypted keys, refer to:
+		Documentation/keys-trusted-encrypted.txt. Both dracut
+		(via 97masterkey and 98integrity) and systemd (via
+		core/ima-setup) have support for loading keys at boot
+		time.

security/integrity/evm/evm.h

@@ -23,6 +23,9 @@
 
 #define EVM_INIT_HMAC	0x0001
 #define EVM_INIT_X509	0x0002
+#define EVM_SETUP       0x80000000 /* userland has signaled key load */
+
+#define EVM_INIT_MASK (EVM_INIT_HMAC | EVM_INIT_X509 | EVM_SETUP)
 
 extern int evm_initialized;
 extern char *evm_hmac;

security/integrity/evm/evm_secfs.c

@@ -40,7 +40,7 @@ static ssize_t evm_read_key(struct file *filp, char __user *buf,
 	if (*ppos != 0)
 		return 0;
 
-	sprintf(temp, "%d", evm_initialized);
+	sprintf(temp, "%d", (evm_initialized & ~EVM_SETUP));
 	rc = simple_read_from_buffer(buf, count, ppos, temp, strlen(temp));
 
 	return rc;
@@ -61,24 +61,29 @@ static ssize_t evm_read_key(struct file *filp, char __user *buf,
 static ssize_t evm_write_key(struct file *file, const char __user *buf,
 			     size_t count, loff_t *ppos)
 {
-	char temp[80];
-	int i;
+	int i, ret;
 
-	if (!capable(CAP_SYS_ADMIN) || (evm_initialized & EVM_INIT_HMAC))
+	if (!capable(CAP_SYS_ADMIN) || (evm_initialized & EVM_SETUP))
 		return -EPERM;
 
-	if (count >= sizeof(temp) || count == 0)
-		return -EINVAL;
-
-	if (copy_from_user(temp, buf, count) != 0)
-		return -EFAULT;
+	ret = kstrtoint_from_user(buf, count, 0, &i);
 
-	temp[count] = '\0';
+	if (ret)
+		return ret;
 
-	if ((sscanf(temp, "%d", &i) != 1) || (i != 1))
+	/* Reject invalid values */
+	if (!i || (i & ~EVM_INIT_MASK) != 0)
 		return -EINVAL;
 
-	evm_init_key();
+	if (i & EVM_INIT_HMAC) {
+		ret = evm_init_key();
+		if (ret != 0)
+			return ret;
+		/* Forbid further writes after the symmetric key is loaded */
+		i |= EVM_SETUP;
+	}
+
+	evm_initialized |= i;
 
 	return count;
 }