kernel/fork.c:copy_process(): don't add the uninitialized child to thread/task/pid lists

copy_process() adds the new child to thread_group/init_task.tasks list and
then does attach_pid(child, PIDTYPE_PID).  This means that the lockless
next_thread() or next_task() can see this thread with the wrong pid.  Say,
"ls /proc/pid/task" can list the same inode twice.

We could move attach_pid(child, PIDTYPE_PID) up, but in this case
find_task_by_vpid() can find the new thread before it was fully
initialized.

And this is already true for PIDTYPE_PGID/PIDTYPE_SID, With this patch
copy_process() initializes child->pids[*].pid first, then calls
attach_pid() to insert the task into the pid->tasks list.

attach_pid() no longer need the "struct pid*" argument, it is always
called after pid_link->pid was already set.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Sergey Dyasly <dserrg@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 19 insertions, 15 deletions

diff --git a/include/linux/pid.h b/include/linux/pid.h
index a089a3c447fc..23705a53abba 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -86,11 +86,9 @@ extern struct task_struct *get_pid_task(struct pid *pid, enum pid_type);
 extern struct pid *get_task_pid(struct task_struct *task, enum pid_type type);
 
 /*
- * attach_pid() and detach_pid() must be called with the tasklist_lock
- * write-held.
+ * these helpers must be called with the tasklist_lock write-held.
  */
-extern void attach_pid(struct task_struct *task, enum pid_type type,
-                        struct pid *pid);
+extern void attach_pid(struct task_struct *task, enum pid_type);
 extern void detach_pid(struct task_struct *task, enum pid_type);
 extern void change_pid(struct task_struct *task, enum pid_type,
                         struct pid *pid);
diff --git a/kernel/fork.c b/kernel/fork.c
index 417cb864e20c..7d6962fb6156 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1121,6 +1121,12 @@ static void posix_cpu_timers_init(struct task_struct *tsk)
         INIT_LIST_HEAD(&tsk->cpu_timers[2]);
 }
 
+static inline void
+init_task_pid(struct task_struct *task, enum pid_type type, struct pid *pid)
+{
+         task->pids[type].pid = pid;
+}
+
 /*
  * This creates a new process as a copy of the old one,
  * but does not actually start it yet.
@@ -1449,7 +1455,11 @@ static struct task_struct *copy_process(unsigned long clone_flags,
         if (likely(p->pid)) {
                 ptrace_init_task(p, (clone_flags & CLONE_PTRACE) || trace);
 
+                init_task_pid(p, PIDTYPE_PID, pid);
                 if (thread_group_leader(p)) {
+                        init_task_pid(p, PIDTYPE_PGID, task_pgrp(current));
+                        init_task_pid(p, PIDTYPE_SID, task_session(current));
+
                         if (is_child_reaper(pid)) {
                                 ns_of_pid(pid)->child_reaper = p;
                                 p->signal->flags |= SIGNAL_UNKILLABLE;
@@ -1457,10 +1467,10 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 
                         p->signal->leader_pid = pid;
                         p->signal->tty = tty_kref_get(current->signal->tty);
-                        attach_pid(p, PIDTYPE_PGID, task_pgrp(current));
-                        attach_pid(p, PIDTYPE_SID, task_session(current));
                         list_add_tail(&p->sibling, &p->real_parent->children);
                         list_add_tail_rcu(&p->tasks, &init_task.tasks);
+                        attach_pid(p, PIDTYPE_PGID);
+                        attach_pid(p, PIDTYPE_SID);
                         __this_cpu_inc(process_counts);
                 } else {
                         current->signal->nr_threads++;
@@ -1470,7 +1480,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
                         list_add_tail_rcu(&p->thread_group,
                                           &p->group_leader->thread_group);
                 }
-                attach_pid(p, PIDTYPE_PID, pid);
+                attach_pid(p, PIDTYPE_PID);
                 nr_threads++;
         }
 
diff --git a/kernel/pid.c b/kernel/pid.c
index 0db3e791a06d..61980cefb1f5 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -373,14 +373,10 @@ EXPORT_SYMBOL_GPL(find_vpid);
 /*
  * attach_pid() must be called with the tasklist_lock write-held.
  */
-void attach_pid(struct task_struct *task, enum pid_type type,
-                struct pid *pid)
+void attach_pid(struct task_struct *task, enum pid_type type)
 {
-        struct pid_link *link;
-
-        link = &task->pids[type];
-        link->pid = pid;
-        hlist_add_head_rcu(&link->node, &pid->tasks[type]);
+        struct pid_link *link = &task->pids[type];
+        hlist_add_head_rcu(&link->node, &link->pid->tasks[type]);
 }
 
 static void __change_pid(struct task_struct *task, enum pid_type type,
@@ -412,7 +408,7 @@ void change_pid(struct task_struct *task, enum pid_type type,
                 struct pid *pid)
 {
         __change_pid(task, type, pid);
-        attach_pid(task, type, pid);
+        attach_pid(task, type);
 }
 
 /* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */