ptrace: fix possible zombie leak on PTRACE_DETACH

When ptrace_detach() takes tasklist, the tracee can be SIGKILL'ed.  If it
has already passed exit_notify() we can leak a zombie, because a) ptracing
disables the auto-reaping logic, and b) ->real_parent was not notified
about the child's death.

ptrace_detach() should follow the ptrace_exit's logic, change the code
accordingly.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Jerome Marchand <jmarchan@redhat.com>
Cc: Roland McGrath <roland@redhat.com>
Tested-by: Denys Vlasenko <dvlasenk@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 8 insertions, 2 deletions

diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index 98b93ca4db06..1a2b0cb55535 100644
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -94,6 +94,7 @@ extern void ptrace_notify(int exit_code);
 extern void __ptrace_link(struct task_struct *child,
                           struct task_struct *new_parent);
 extern void __ptrace_unlink(struct task_struct *child);
+extern int __ptrace_detach(struct task_struct *tracer, struct task_struct *p);
 extern void ptrace_fork(struct task_struct *task, unsigned long clone_flags);
 #define PTRACE_MODE_READ   1
 #define PTRACE_MODE_ATTACH 2
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index f62a568e84ec..ee553b6ad125 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -237,6 +237,8 @@ out:
 
 int ptrace_detach(struct task_struct *child, unsigned int data)
 {
+        int dead = 0;
+
         if (!valid_signal(data))
                 return -EIO;
 
@@ -244,18 +246,21 @@ int ptrace_detach(struct task_struct *child, unsigned int data)
         ptrace_disable(child);
         clear_tsk_thread_flag(child, TIF_SYSCALL_TRACE);
 
-        /* protect against de_thread()->release_task() */
         write_lock_irq(&tasklist_lock);
+        /* protect against de_thread()->release_task() */
         if (child->ptrace) {
                 child->exit_code = data;
 
-                __ptrace_unlink(child);
+                dead = __ptrace_detach(current, child);
 
                 if (!child->exit_state)
                         wake_up_process(child);
         }
         write_unlock_irq(&tasklist_lock);
 
+        if (unlikely(dead))
+                release_task(child);
+
         return 0;
 }