[PATCH] Keys: Pass session keyring to call_usermodehelper()

The attached patch makes it possible to pass a session keyring through to the
process spawned by call_usermodehelper(). This allows patch 3/3 to pass an
authorisation key through to /sbin/request-key, thus permitting better access
controls when doing just-in-time key creation.

Signed-Off-By: David Howells <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/key.h

···
 extern void key_fsgid_changed(struct task_struct *tsk);
 extern void key_init(void);
 
+#define __install_session_keyring(tsk, keyring)			\
+({								\
+	struct key *old_session = tsk->signal->session_keyring;	\
+	tsk->signal->session_keyring = keyring;			\
+	old_session;						\
+})
+
 #else /* CONFIG_KEYS */
 
 #define key_validate(k)			0
 #define key_serial(k)			0
-#define key_get(k) 			NULL
+#define key_get(k) 			({ NULL; })
 #define key_put(k)			do { } while(0)
 #define alloc_uid_keyring(u)		0
 #define switch_uid_keyring(u)		do { } while(0)
+#define __install_session_keyring(t, k)	({ NULL; })
 #define copy_keys(f,t)			0
 #define copy_thread_group_keys(t)	0
 #define exit_keys(t)			do { } while(0)

include/linux/kmod.h

···
  *      Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  */
 
+#include <linux/stddef.h>
 #include <linux/config.h>
 #include <linux/errno.h>
 #include <linux/compiler.h>
···
 #endif
 
 #define try_then_request_module(x, mod...) ((x) ?: (request_module(mod), (x)))
-extern int call_usermodehelper(char *path, char *argv[], char *envp[], int wait);
+
+struct key;
+extern int call_usermodehelper_keys(char *path, char *argv[], char *envp[],
+				    struct key *session_keyring, int wait);
+
+static inline int
+call_usermodehelper(char *path, char **argv, char **envp, int wait)
+{
+	return call_usermodehelper_keys(path, argv, envp, NULL, wait);
+}
+
 extern void usermodehelper_init(void);
 
 #endif /* __LINUX_KMOD_H__ */

kernel/kmod.c

···
 	char *path;
 	char **argv;
 	char **envp;
+	struct key *ring;
 	int wait;
 	int retval;
 };
···
 static int ____call_usermodehelper(void *data)
 {
 	struct subprocess_info *sub_info = data;
+	struct key *old_session;
 	int retval;
 
-	/* Unblock all signals. */
+	/* Unblock all signals and set the session keyring. */
+	key_get(sub_info->ring);
 	flush_signals(current);
 	spin_lock_irq(&current->sighand->siglock);
+	old_session = __install_session_keyring(current, sub_info->ring);
 	flush_signal_handlers(current, 1);
 	sigemptyset(&current->blocked);
 	recalc_sigpending();
 	spin_unlock_irq(&current->sighand->siglock);
+
+	key_put(old_session);
 
 	/* We can run anywhere, unlike our parent keventd(). */
 	set_cpus_allowed(current, CPU_MASK_ALL);
···
 }
 
 /**
- * call_usermodehelper - start a usermode application
+ * call_usermodehelper_keys - start a usermode application
  * @path: pathname for the application
  * @argv: null-terminated argument list
  * @envp: null-terminated environment list
+ * @session_keyring: session keyring for process (NULL for an empty keyring)
  * @wait: wait for the application to finish and return status.
  *
  * Runs a user-space application.  The application is started
···
  * Must be called from process context.  Returns a negative error code
  * if program was not execed successfully, or 0.
  */
-int call_usermodehelper(char *path, char **argv, char **envp, int wait)
+int call_usermodehelper_keys(char *path, char **argv, char **envp,
+			     struct key *session_keyring, int wait)
 {
 	DECLARE_COMPLETION(done);
 	struct subprocess_info sub_info = {
···
 		.path		= path,
 		.argv		= argv,
 		.envp		= envp,
+		.ring		= session_keyring,
 		.wait		= wait,
 		.retval		= 0,
 	};
···
 	wait_for_completion(&done);
 	return sub_info.retval;
 }
-EXPORT_SYMBOL(call_usermodehelper);
+EXPORT_SYMBOL(call_usermodehelper_keys);
 
 void __init usermodehelper_init(void)
 {

security/keys/request_key.c

···
 	argv[i] = NULL;
 
 	/* do it */
-	return call_usermodehelper(argv[0], argv, envp, 1);
+	return call_usermodehelper_keys(argv[0], argv, envp, NULL, 1);
 
 } /* end call_request_key() */