[PATCH] fix de_thread() vs send_group_sigqueue() race

When non-leader thread does exec, de_thread calls release_task(leader) before
calling exit_itimers(). If local timer interrupt happens in between, it can
oops in send_group_sigqueue() while taking ->sighand->siglock == NULL.

However, we can't change send_group_sigqueue() to check p->signal != NULL,
because sys_timer_create() does get_task_struct() only in SIGEV_THREAD_ID
case. So it is possible that this task_struct was already freed and we can't
trust p->signal.

This patch changes de_thread() so that leader released after exit_itimers()
call.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Chris Wright <chrisw@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/fs/exec.c b/fs/exec.c
index cd6c574..5a4e3ac 100644 (file)
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -590,6 +590,7 @@ static inline int de_thread(struct task_struct *tsk)
        struct signal_struct *sig = tsk->signal;
        struct sighand_struct *newsighand, *oldsighand = tsk->sighand;
        spinlock_t *lock = &oldsighand->siglock;
+       struct task_struct *leader = NULL;
        int count;
 
        /*
@@ -665,7 +666,7 @@ static inline int de_thread(struct task_struct *tsk)
         * and to assume its PID:
         */
        if (!thread_group_leader(current)) {
-               struct task_struct *leader = current->group_leader, *parent;
+               struct task_struct *parent;
                struct dentry *proc_dentry1, *proc_dentry2;
                unsigned long exit_state, ptrace;
 
@@ -674,6 +675,7 @@ static inline int de_thread(struct task_struct *tsk)
                 * It should already be zombie at this point, most
                 * of the time.
                 */
+               leader = current->group_leader;
                while (leader->exit_state != EXIT_ZOMBIE)
                        yield();
 
@@ -733,7 +735,6 @@ static inline int de_thread(struct task_struct *tsk)
                proc_pid_flush(proc_dentry2);
 
                BUG_ON(exit_state != EXIT_ZOMBIE);
-               release_task(leader);
         }
 
        /*
@@ -743,8 +744,11 @@ static inline int de_thread(struct task_struct *tsk)
        sig->flags = 0;
 
 no_thread_group:
-       BUG_ON(atomic_read(&sig->count) != 1);
        exit_itimers(sig);
+       if (leader)
+               release_task(leader);
+
+       BUG_ON(atomic_read(&sig->count) != 1);
 
        if (atomic_read(&oldsighand->count) == 1) {
                /*