pids: introduce change_pid() helper

Based on Eric W. Biederman's idea.

Without tasklist_lock held task_session()/task_pgrp() can return NULL if the
caller races with setprgp()/setsid() which does detach_pid() + attach_pid().
This can happen even if task == current.

Intoduce the new helper, change_pid(), which should be used instead.  This way
the caller always sees the special pid != NULL, either old or new.

Also change the prototype of attach_pid(), it always returns 0 and nobody
check the returned value.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc:  "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 16 insertions, 5 deletions

diff --git a/kernel/pid.c b/kernel/pid.c
index e9a31d362b28..20d59fa2d493 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -317,7 +317,7 @@ EXPORT_SYMBOL_GPL(find_pid);
 /*
  * attach_pid() must be called with the tasklist_lock write-held.
  */
-int attach_pid(struct task_struct *task, enum pid_type type,
+void attach_pid(struct task_struct *task, enum pid_type type,
                 struct pid *pid)
 {
         struct pid_link *link;
@@ -325,11 +325,10 @@ int attach_pid(struct task_struct *task, enum pid_type type,
         link = &task->pids[type];
         link->pid = pid;
         hlist_add_head_rcu(&link->node, &pid->tasks[type]);
-
-        return 0;
 }
 
-void detach_pid(struct task_struct *task, enum pid_type type)
+static void __change_pid(struct task_struct *task, enum pid_type type,
+                        struct pid *new)
 {
         struct pid_link *link;
         struct pid *pid;
@@ -339,7 +338,7 @@ void detach_pid(struct task_struct *task, enum pid_type type)
         pid = link->pid;
 
         hlist_del_rcu(&link->node);
-        link->pid = NULL;
+        link->pid = new;
 
         for (tmp = PIDTYPE_MAX; --tmp >= 0; )
                 if (!hlist_empty(&pid->tasks[tmp]))
@@ -348,6 +347,18 @@ void detach_pid(struct task_struct *task, enum pid_type type)
         free_pid(pid);
 }
 
+void detach_pid(struct task_struct *task, enum pid_type type)
+{
+        __change_pid(task, type, NULL);
+}
+
+void change_pid(struct task_struct *task, enum pid_type type,
+                struct pid *pid)
+{
+        __change_pid(task, type, pid);
+        attach_pid(task, type, pid);
+}
+
 /* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
 void transfer_pid(struct task_struct *old, struct task_struct *new,
                            enum pid_type type)