keys: add keyctl function to get a security label

Add a keyctl() function to get the security label of a key.

The following is added to Documentation/keys.txt:

(*) Get the LSM security context attached to a key.

long keyctl(KEYCTL_GET_SECURITY, key_serial_t key, char *buffer,
size_t buflen)

This function returns a string that represents the LSM security context
attached to a key in the buffer provided.

Unless there's an error, it always returns the amount of data it could
produce, even if that's too big for the buffer, but it won't copy more
than requested to userspace. If the buffer pointer is NULL then no copy
will take place.

A NUL character is included at the end of the string if the buffer is
sufficiently big. This is included in the returned count. If no LSM is
in force then an empty string will be returned.

A process must have view permission on the key for this function to be
successful.

[[e-mail]: declare keyctl_get_security()]
Signed-off-by: David Howells <[e-mail]>
Acked-by: Stephen Smalley <[e-mail]>
Cc: Paul Moore <[e-mail]>
Cc: Chris Wright <[e-mail]>
Cc: James Morris <[e-mail]>
Cc: Kevin Coffman <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/keys.txt

···
      The assumed authoritative key is inherited across fork and exec.
 
 
+ (*) Get the LSM security context attached to a key.
+
+	long keyctl(KEYCTL_GET_SECURITY, key_serial_t key, char *buffer,
+		    size_t buflen)
+
+     This function returns a string that represents the LSM security context
+     attached to a key in the buffer provided.
+
+     Unless there's an error, it always returns the amount of data it could
+     produce, even if that's too big for the buffer, but it won't copy more
+     than requested to userspace. If the buffer pointer is NULL then no copy
+     will take place.
+
+     A NUL character is included at the end of the string if the buffer is
+     sufficiently big.  This is included in the returned count.  If no LSM is
+     in force then an empty string will be returned.
+
+     A process must have view permission on the key for this function to be
+     successful.
+
+
 ===============
 KERNEL SERVICES
 ===============

include/linux/keyctl.h

···
 #define KEYCTL_SET_REQKEY_KEYRING	14	/* set default request-key keyring */
 #define KEYCTL_SET_TIMEOUT		15	/* set key timeout */
 #define KEYCTL_ASSUME_AUTHORITY		16	/* assume request_key() authorisation */
+#define KEYCTL_GET_SECURITY		17	/* get key security label */
 
 #endif /*  _LINUX_KEYCTL_H */

include/linux/security.h

···
  *	@perm describes the combination of permissions required of this key.
  *	Return 1 if permission granted, 0 if permission denied and -ve it the
  *	normal permissions model should be effected.
+ * @key_getsecurity:
+ *	Get a textual representation of the security context attached to a key
+ *	for the purposes of honouring KEYCTL_GETSECURITY.  This function
+ *	allocates the storage for the NUL-terminated string and the caller
+ *	should free it.
+ *	@key points to the key to be queried.
+ *	@_buffer points to a pointer that should be set to point to the
+ *	 resulting string (if no label or an error occurs).
+ *	Return the length of the string (including terminating NUL) or -ve if
+ *      an error.
+ *	May also return 0 (and a NULL buffer pointer) if there is no label.
  *
  * Security hooks affecting all System V IPC operations.
  *
···
 	int (*key_permission) (key_ref_t key_ref,
 			       struct task_struct *context,
 			       key_perm_t perm);
-
+	int (*key_getsecurity)(struct key *key, char **_buffer);
 #endif	/* CONFIG_KEYS */
 
 #ifdef CONFIG_AUDIT
···
 void security_key_free(struct key *key);
 int security_key_permission(key_ref_t key_ref,
 			    struct task_struct *context, key_perm_t perm);
+int security_key_getsecurity(struct key *key, char **_buffer);
 
 #else
 
···
 					  struct task_struct *context,
 					  key_perm_t perm)
 {
+	return 0;
+}
+
+static inline int security_key_getsecurity(struct key *key, char **_buffer)
+{
+	*_buffer = NULL;
 	return 0;
 }
 

security/dummy.c

···
 {
 	return 0;
 }
+
+static int dummy_key_getsecurity(struct key *key, char **_buffer)
+{
+	*_buffer = NULL;
+	return 0;
+}
+
 #endif /* CONFIG_KEYS */
 
 #ifdef CONFIG_AUDIT
···
 	set_to_dummy_if_null(ops, key_alloc);
 	set_to_dummy_if_null(ops, key_free);
 	set_to_dummy_if_null(ops, key_permission);
+	set_to_dummy_if_null(ops, key_getsecurity);
 #endif	/* CONFIG_KEYS */
 #ifdef CONFIG_AUDIT
 	set_to_dummy_if_null(ops, audit_rule_init);

security/keys/compat.c

···
 	case KEYCTL_ASSUME_AUTHORITY:
 		return keyctl_assume_authority(arg2);
 
+	case KEYCTL_GET_SECURITY:
+		return keyctl_get_security(arg2, compat_ptr(arg3), arg4);
+
 	default:
 		return -EOPNOTSUPP;
 	}

security/keys/internal.h

···
 extern long keyctl_set_reqkey_keyring(int);
 extern long keyctl_set_timeout(key_serial_t, unsigned);
 extern long keyctl_assume_authority(key_serial_t);
-
+extern long keyctl_get_security(key_serial_t keyid, char __user *buffer,
+				size_t buflen);
 
 /*
  * debugging key validation

security/keys/keyctl.c

···
 #include <linux/string.h>
 #include <linux/err.h>
 #include <linux/vmalloc.h>
+#include <linux/security.h>
 #include <asm/uaccess.h>
 #include "internal.h"
 
···
 
 } /* end keyctl_assume_authority() */
 
+/*
+ * get the security label of a key
+ * - the key must grant us view permission
+ * - if there's a buffer, we place up to buflen bytes of data into it
+ * - unless there's an error, we return the amount of information available,
+ *   irrespective of how much we may have copied (including the terminal NUL)
+ * - implements keyctl(KEYCTL_GET_SECURITY)
+ */
+long keyctl_get_security(key_serial_t keyid,
+			 char __user *buffer,
+			 size_t buflen)
+{
+	struct key *key, *instkey;
+	key_ref_t key_ref;
+	char *context;
+	long ret;
+
+	key_ref = lookup_user_key(NULL, keyid, 0, 1, KEY_VIEW);
+	if (IS_ERR(key_ref)) {
+		if (PTR_ERR(key_ref) != -EACCES)
+			return PTR_ERR(key_ref);
+
+		/* viewing a key under construction is also permitted if we
+		 * have the authorisation token handy */
+		instkey = key_get_instantiation_authkey(keyid);
+		if (IS_ERR(instkey))
+			return PTR_ERR(key_ref);
+		key_put(instkey);
+
+		key_ref = lookup_user_key(NULL, keyid, 0, 1, 0);
+		if (IS_ERR(key_ref))
+			return PTR_ERR(key_ref);
+	}
+
+	key = key_ref_to_ptr(key_ref);
+	ret = security_key_getsecurity(key, &context);
+	if (ret == 0) {
+		/* if no information was returned, give userspace an empty
+		 * string */
+		ret = 1;
+		if (buffer && buflen > 0 &&
+		    copy_to_user(buffer, "", 1) != 0)
+			ret = -EFAULT;
+	} else if (ret > 0) {
+		/* return as much data as there's room for */
+		if (buffer && buflen > 0) {
+			if (buflen > ret)
+				buflen = ret;
+
+			if (copy_to_user(buffer, context, buflen) != 0)
+				ret = -EFAULT;
+		}
+
+		kfree(context);
+	}
+
+	key_ref_put(key_ref);
+	return ret;
+}
+
 /*****************************************************************************/
 /*
  * the key control system call
···
 
 	case KEYCTL_ASSUME_AUTHORITY:
 		return keyctl_assume_authority((key_serial_t) arg2);
+
+	case KEYCTL_GET_SECURITY:
+		return keyctl_get_security((key_serial_t) arg2,
+					   (char *) arg3,
+					   (size_t) arg4);
 
 	default:
 		return -EOPNOTSUPP;

security/security.c

···
 	return security_ops->key_permission(key_ref, context, perm);
 }
 
+int security_key_getsecurity(struct key *key, char **_buffer)
+{
+	return security_ops->key_getsecurity(key, _buffer);
+}
+
 #endif	/* CONFIG_KEYS */
 
 #ifdef CONFIG_AUDIT

security/selinux/hooks.c

···
 			    SECCLASS_KEY, perm, NULL);
 }
 
+static int selinux_key_getsecurity(struct key *key, char **_buffer)
+{
+	struct key_security_struct *ksec = key->security;
+	char *context = NULL;
+	unsigned len;
+	int rc;
+
+	rc = security_sid_to_context(ksec->sid, &context, &len);
+	if (!rc)
+		rc = len;
+	*_buffer = context;
+	return rc;
+}
+
 #endif
 
 static struct security_operations selinux_ops = {
···
 	.key_alloc =			selinux_key_alloc,
 	.key_free =			selinux_key_free,
 	.key_permission =		selinux_key_permission,
+	.key_getsecurity =		selinux_key_getsecurity,
 #endif
 
 #ifdef CONFIG_AUDIT