umh: creds: convert call_usermodehelper_keys() to use subprocess_info->init()

call_usermodehelper_keys() uses call_usermodehelper_setkeys() to change
subprocess_info->cred in advance. Now that we have info->init() we can
change this code to set tgcred->session_keyring in context of execing
kernel thread.

Note: since currently call_usermodehelper_keys() is never called with
UMH_NO_WAIT, call_usermodehelper_keys()->key_get() and umh_keys_cleanup()
are not really needed, we could rely on install_session_keyring_to_cred()
which does key_get() on success.

Signed-off-by: Oleg Nesterov <[e-mail]>
Acked-by: Neil Horman <[e-mail]>
Acked-by: David Howells <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/kmod.h

···
 						  char **envp, gfp_t gfp_mask);
 
 /* Set various pieces of state into the subprocess_info structure */
-void call_usermodehelper_setkeys(struct subprocess_info *info,
-				 struct key *session_keyring);
 void call_usermodehelper_setfns(struct subprocess_info *info,
 		    int (*init)(struct subprocess_info *info),
 		    void (*cleanup)(struct subprocess_info *info),
···
 {
 	return call_usermodehelper_fns(path, argv, envp, wait,
 				       NULL, NULL, NULL);
-}
-
-static inline int
-call_usermodehelper_keys(char *path, char **argv, char **envp,
-			 struct key *session_keyring, enum umh_wait wait)
-{
-	struct subprocess_info *info;
-	gfp_t gfp_mask = (wait == UMH_NO_WAIT) ? GFP_ATOMIC : GFP_KERNEL;
-
-	info = call_usermodehelper_setup(path, argv, envp, gfp_mask);
-	if (info == NULL)
-		return -ENOMEM;
-
-	call_usermodehelper_setkeys(info, session_keyring);
-	return call_usermodehelper_exec(info, wait);
 }
 
 extern void usermodehelper_init(void);

kernel/kmod.c

···
 EXPORT_SYMBOL(call_usermodehelper_setup);
 
 /**
- * call_usermodehelper_setkeys - set the session keys for usermode helper
- * @info: a subprocess_info returned by call_usermodehelper_setup
- * @session_keyring: the session keyring for the process
- */
-void call_usermodehelper_setkeys(struct subprocess_info *info,
-				 struct key *session_keyring)
-{
-#ifdef CONFIG_KEYS
-	struct thread_group_cred *tgcred = info->cred->tgcred;
-	key_put(tgcred->session_keyring);
-	tgcred->session_keyring = key_get(session_keyring);
-#else
-	BUG();
-#endif
-}
-EXPORT_SYMBOL(call_usermodehelper_setkeys);
-
-/**
  * call_usermodehelper_setfns - set a cleanup/init function
  * @info: a subprocess_info returned by call_usermodehelper_setup
  * @cleanup: a cleanup function

security/keys/internal.h

···
 extern int install_user_keyrings(void);
 extern int install_thread_keyring_to_cred(struct cred *);
 extern int install_process_keyring_to_cred(struct cred *);
+extern int install_session_keyring_to_cred(struct cred *, struct key *);
 
 extern struct key *request_key_and_link(struct key_type *type,
 					const char *description,

security/keys/process_keys.c

···
 /*
  * install a session keyring directly to a credentials struct
  */
-static int install_session_keyring_to_cred(struct cred *cred,
-					   struct key *keyring)
+int install_session_keyring_to_cred(struct cred *cred, struct key *keyring)
 {
 	unsigned long flags;
 	struct key *old;

security/keys/request_key.c

···
 }
 EXPORT_SYMBOL(complete_request_key);
 
+static int umh_keys_init(struct subprocess_info *info)
+{
+	struct cred *cred = (struct cred*)current_cred();
+	struct key *keyring = info->data;
+	/*
+	 * This is called in context of freshly forked kthread before
+	 * kernel_execve(), we can just change our ->session_keyring.
+	 */
+	return install_session_keyring_to_cred(cred, keyring);
+}
+
+static void umh_keys_cleanup(struct subprocess_info *info)
+{
+	struct key *keyring = info->data;
+	key_put(keyring);
+}
+
+static int call_usermodehelper_keys(char *path, char **argv, char **envp,
+			 struct key *session_keyring, enum umh_wait wait)
+{
+	gfp_t gfp_mask = (wait == UMH_NO_WAIT) ? GFP_ATOMIC : GFP_KERNEL;
+	struct subprocess_info *info =
+		call_usermodehelper_setup(path, argv, envp, gfp_mask);
+
+	if (!info)
+		return -ENOMEM;
+
+	call_usermodehelper_setfns(info, umh_keys_init, umh_keys_cleanup,
+					key_get(session_keyring));
+	return call_usermodehelper_exec(info, wait);
+}
+
 /*
  * request userspace finish the construction of a key
  * - execute "/sbin/request-key <op> <key> <uid> <gid> <keyring> <keyring> <keyring>"