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>

1 files changed, 10 insertions, 9 deletions

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index f6d8b8cb5e34..9439bd3331a6 100644
--- 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);