[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>

1 files changed, 7 insertions, 3 deletions

diff --git a/fs/exec.c b/fs/exec.c
index cd6c574557dc..5a4e3acc2e9f 100644
--- 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) {
                 /*