signals: move cred_guard_mutex from task_struct to signal_struct

Oleg Nesterov pointed out we have to prevent multiple-threads-inside-exec
itself and we can reuse ->cred_guard_mutex for it.  Yes, concurrent
execve() has no worth.

Let's move ->cred_guard_mutex from task_struct to signal_struct.  It
naturally prevent multiple-threads-inside-exec.
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Roland McGrath <roland@redhat.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/exec.c

@@ -1083,14 +1083,14 @@ EXPORT_SYMBOL(setup_new_exec);
  */
 int prepare_bprm_creds(struct linux_binprm *bprm)
 {
-	if (mutex_lock_interruptible(&current->cred_guard_mutex))
+	if (mutex_lock_interruptible(&current->signal->cred_guard_mutex))
 		return -ERESTARTNOINTR;
 
 	bprm->cred = prepare_exec_creds();
 	if (likely(bprm->cred))
 		return 0;
 
-	mutex_unlock(&current->cred_guard_mutex);
+	mutex_unlock(&current->signal->cred_guard_mutex);
 	return -ENOMEM;
 }
 
@@ -1098,7 +1098,7 @@ void free_bprm(struct linux_binprm *bprm)
 {
 	free_arg_pages(bprm);
 	if (bprm->cred) {
-		mutex_unlock(&current->cred_guard_mutex);
+		mutex_unlock(&current->signal->cred_guard_mutex);
 		abort_creds(bprm->cred);
 	}
 	kfree(bprm);
@@ -1119,13 +1119,13 @@ void install_exec_creds(struct linux_binprm *bprm)
 	 * credentials; any time after this it may be unlocked.
 	 */
 	security_bprm_committed_creds(bprm);
-	mutex_unlock(&current->cred_guard_mutex);
+	mutex_unlock(&current->signal->cred_guard_mutex);
 }
 EXPORT_SYMBOL(install_exec_creds);
 
 /*
  * determine how safe it is to execute the proposed program
- * - the caller must hold current->cred_guard_mutex to protect against
+ * - the caller must hold ->cred_guard_mutex to protect against
  *   PTRACE_ATTACH
  */
 int check_unsafe_exec(struct linux_binprm *bprm)

fs/proc/base.c

@@ -226,7 +226,7 @@ struct mm_struct *mm_for_maps(struct task_struct *task)
 {
 	struct mm_struct *mm;
 
-	if (mutex_lock_killable(&task->cred_guard_mutex))
+	if (mutex_lock_killable(&task->signal->cred_guard_mutex))
 		return NULL;
 
 	mm = get_task_mm(task);
@@ -235,7 +235,7 @@ struct mm_struct *mm_for_maps(struct task_struct *task)
 		mmput(mm);
 		mm = NULL;
 	}
-	mutex_unlock(&task->cred_guard_mutex);
+	mutex_unlock(&task->signal->cred_guard_mutex);
 
 	return mm;
 }
@@ -2354,14 +2354,14 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
 		goto out_free;
 
 	/* Guard against adverse ptrace interaction */
-	length = mutex_lock_interruptible(&task->cred_guard_mutex);
+	length = mutex_lock_interruptible(&task->signal->cred_guard_mutex);
 	if (length < 0)
 		goto out_free;
 
 	length = security_setprocattr(task,
 				      (char*)file->f_path.dentry->d_name.name,
 				      (void*)page, count);
-	mutex_unlock(&task->cred_guard_mutex);
+	mutex_unlock(&task->signal->cred_guard_mutex);
 out_free:
 	free_page((unsigned long) page);
 out:

include/linux/init_task.h

@@ -29,6 +29,8 @@ extern struct fs_struct init_fs;
 		.running = 0,						\
 		.lock = __SPIN_LOCK_UNLOCKED(sig.cputimer.lock),	\
 	},								\
+	.cred_guard_mutex =						\
+		 __MUTEX_INITIALIZER(sig.cred_guard_mutex),		\
 }
 
 extern struct nsproxy init_nsproxy;
@@ -145,8 +147,6 @@ extern struct cred init_cred;
 	.group_leader	= &tsk,						\
 	RCU_INIT_POINTER(.real_cred, &init_cred),			\
 	RCU_INIT_POINTER(.cred, &init_cred),				\
-	.cred_guard_mutex =						\
-		 __MUTEX_INITIALIZER(tsk.cred_guard_mutex),		\
 	.comm		= "swapper",					\
 	.thread		= INIT_THREAD,					\
 	.fs		= &init_fs,					\

include/linux/sched.h

@@ -626,6 +626,10 @@ struct signal_struct {
 
 	int oom_adj;		/* OOM kill score adjustment (bit shift) */
 	int oom_score_adj;	/* OOM kill score adjustment */
+
+	struct mutex cred_guard_mutex;	/* guard against foreign influences on
+					 * credential calculations
+					 * (notably. ptrace) */
 };
 
 /* Context switch must be unlocked if interrupts are to be enabled */
@@ -1305,9 +1309,6 @@ struct task_struct {
 					 * credentials (COW) */
 	const struct cred __rcu *cred;	/* effective (overridable) subjective task
 					 * credentials (COW) */
-	struct mutex cred_guard_mutex;	/* guard against foreign influences on
-					 * credential calculations
-					 * (notably. ptrace) */
 	struct cred *replacement_session_keyring; /* for KEYCTL_SESSION_TO_PARENT */
 
 	char comm[TASK_COMM_LEN]; /* executable name excluding path

include/linux/tracehook.h

@@ -150,7 +150,7 @@ static inline void tracehook_report_syscall_exit(struct pt_regs *regs, int step)
  *
  * Return %LSM_UNSAFE_* bits applied to an exec because of tracing.
  *
- * @task->cred_guard_mutex is held by the caller through the do_execve().
+ * @task->signal->cred_guard_mutex is held by the caller through the do_execve().
  */
 static inline int tracehook_unsafe_exec(struct task_struct *task)
 {

kernel/cred.c

@@ -325,7 +325,7 @@ EXPORT_SYMBOL(prepare_creds);
 
 /*
  * Prepare credentials for current to perform an execve()
- * - The caller must hold current->cred_guard_mutex
+ * - The caller must hold ->cred_guard_mutex
  */
 struct cred *prepare_exec_creds(void)
 {
@@ -384,8 +384,6 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
 	struct cred *new;
 	int ret;
 
-	mutex_init(&p->cred_guard_mutex);
-
 	if (
 #ifdef CONFIG_KEYS
 		!p->cred->thread_keyring &&

kernel/fork.c

@@ -908,6 +908,8 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
 	sig->oom_adj = current->signal->oom_adj;
 	sig->oom_score_adj = current->signal->oom_score_adj;
 
+	mutex_init(&sig->cred_guard_mutex);
+
 	return 0;
 }
 

kernel/ptrace.c

@@ -181,7 +181,7 @@ int ptrace_attach(struct task_struct *task)
 	 * under ptrace.
 	 */
 	retval = -ERESTARTNOINTR;
-	if (mutex_lock_interruptible(&task->cred_guard_mutex))
+	if (mutex_lock_interruptible(&task->signal->cred_guard_mutex))
 		goto out;
 
 	task_lock(task);
@@ -208,7 +208,7 @@ int ptrace_attach(struct task_struct *task)
 unlock_tasklist:
 	write_unlock_irq(&tasklist_lock);
 unlock_creds:
-	mutex_unlock(&task->cred_guard_mutex);
+	mutex_unlock(&task->signal->cred_guard_mutex);
 out:
 	return retval;
 }