ptrace: ptrace_attach: check PF_KTHREAD + exit_state instead of ->mm

- Add PF_KTHREAD check to prevent attaching to the kernel thread
  with a borrowed ->mm.

  With or without this change we can race with daemonize() which
  can set PF_KTHREAD or clear ->mm after ptrace_attach() does the
  check, but this doesn't matter because reparent_to_kthreadd()
  does ptrace_unlink().

- Kill "!task->mm" check. We don't really care about ->mm != NULL,
  and the task can call exit_mm() right after we drop task_lock().
  What we need is to make sure we can't attach after exit_notify(),
  check task->exit_state != 0 instead.

Also, move the "already traced" check down for cosmetic reasons.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Chris Wright <chrisw@sous-sol.org>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index f6d8b8c..9439bd3 100644 (file)
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -172,6 +172,8 @@ int ptrace_attach(struct task_struct *task)
        audit_ptrace(task);
 
        retval = -EPERM;
+       if (unlikely(task->flags & PF_KTHREAD))
+               goto out;
        if (same_thread_group(task, current))
                goto out;
 
@@ -182,8 +184,6 @@ int ptrace_attach(struct task_struct *task)
        retval = mutex_lock_interruptible(&task->cred_guard_mutex);
        if (retval  < 0)
                goto out;
-
-       retval = -EPERM;
 repeat:
        /*
         * Nasty, nasty.
@@ -203,23 +203,24 @@ repeat:
                goto repeat;
        }
 
-       if (!task->mm)
-               goto bad;
-       /* the same process cannot be attached many times */
-       if (task->ptrace & PT_PTRACED)
-               goto bad;
        retval = __ptrace_may_access(task, PTRACE_MODE_ATTACH);
        if (retval)
                goto bad;
 
-       /* Go */
+       retval = -EPERM;
+       if (unlikely(task->exit_state))
+               goto bad;
+       if (task->ptrace & PT_PTRACED)
+               goto bad;
+
        task->ptrace |= PT_PTRACED;
        if (capable(CAP_SYS_PTRACE))
                task->ptrace |= PT_PTRACE_CAP;
 
        __ptrace_link(task, current);
-
        send_sig_info(SIGSTOP, SEND_SIG_FORCED, task);
+
+       retval = 0;
 bad:
        write_unlock_irqrestore(&tasklist_lock, flags);
        task_unlock(task);