Fix ptrace_attach()/ptrace_traceme()/de_thread() race

This holds the task lock (and, for ptrace_attach, the tasklist_lock)
over the actual attach event, which closes a race between attacking to a
thread that is either doing a PTRACE_TRACEME or getting de-threaded.

Thanks to Oleg Nesterov for reminding me about this, and Chris Wright
for noticing a lost return value in my first version.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 21 insertions, 18 deletions

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 4e0f0ec003f7..b0f8da80d7d4 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -148,12 +148,16 @@ int ptrace_may_attach(struct task_struct *task)
 int ptrace_attach(struct task_struct *task)
 {
         int retval;
-        task_lock(task);
+
         retval = -EPERM;
         if (task->pid <= 1)
-                goto bad;
+                goto out;
         if (task->tgid == current->tgid)
-                goto bad;
+                goto out;
+
+        write_lock_irq(&tasklist_lock);
+        task_lock(task);
+
         /* the same process cannot be attached many times */
         if (task->ptrace & PT_PTRACED)
                 goto bad;
@@ -166,17 +170,15 @@ int ptrace_attach(struct task_struct *task)
                                       ? PT_ATTACHED : 0);
         if (capable(CAP_SYS_PTRACE))
                 task->ptrace |= PT_PTRACE_CAP;
-        task_unlock(task);
 
-        write_lock_irq(&tasklist_lock);
         __ptrace_link(task, current);
-        write_unlock_irq(&tasklist_lock);
 
         force_sig_specific(SIGSTOP, task);
-        return 0;
 
 bad:
+        write_unlock_irq(&tasklist_lock);
         task_unlock(task);
+out:
         return retval;
 }
 
@@ -417,21 +419,22 @@ int ptrace_request(struct task_struct *child, long request,
  */
 int ptrace_traceme(void)
 {
-        int ret;
+        int ret = -EPERM;
 
         /*
          * Are we already being traced?
          */
-        if (current->ptrace & PT_PTRACED)
-                return -EPERM;
-        ret = security_ptrace(current->parent, current);
-        if (ret)
-                return -EPERM;
-        /*
-         * Set the ptrace bit in the process ptrace flags.
-         */
-        current->ptrace |= PT_PTRACED;
-        return 0;
+        task_lock(current);
+        if (!(current->ptrace & PT_PTRACED)) {
+                ret = security_ptrace(current->parent, current);
+                /*
+                 * Set the ptrace bit in the process ptrace flags.
+                 */
+                if (!ret)
+                        current->ptrace |= PT_PTRACED;
+        }
+        task_unlock(current);
+        return ret;
 }
 
 /**