[PATCH] Fix atomicity of TIF update in flush_thread() for x86_64

Fix atomicity of TIF update in flush_thread() for x86_64

Race :

parent process executing :
sys_ptrace()
 (lock_kernel())
 (ptrace_get_task_struct(pid))
 arch_ptrace()
   ptrace_detach()
     ptrace_disable(child);
       clear_singlestep(child);
         clear_tsk_thread_flag(child, TIF_SINGLESTEP);
         (which clears the TIF_SINGLESTEP flag atomically from a different
	  process)
 (put_task_struct(child))
 (unlock_kernel())

And at the same time, in the child process :
sys_execve()
 do_execve()
   search_binary_handler()
     load_elf_binary()
       flush_old_exec()
         flush_thread()
           doing a non-atomic thread flag update
Signed-off-by: NRebecca Schultz <rschultz@google.com>
Signed-off-by: NMathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Acked-by: NAndi Kleen <ak@suse.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/x86_64/kernel/process.c

@@ -382,14 +382,17 @@ void exit_thread(void)
 void flush_thread(void)
 {
 	struct task_struct *tsk = current;
-	struct thread_info *t = current_thread_info();
 
-	if (t->flags & _TIF_ABI_PENDING) {
-		t->flags ^= (_TIF_ABI_PENDING | _TIF_IA32);
-		if (t->flags & _TIF_IA32)
+	if (test_tsk_thread_flag(tsk, TIF_ABI_PENDING)) {
+		clear_tsk_thread_flag(tsk, TIF_ABI_PENDING);
+		if (test_tsk_thread_flag(tsk, TIF_IA32)) {
+			clear_tsk_thread_flag(tsk, TIF_IA32);
+		} else {
+			set_tsk_thread_flag(tsk, TIF_IA32);
 			current_thread_info()->status |= TS_COMPAT;
+		}
 	}
-	t->flags &= ~_TIF_DEBUG;
+	clear_tsk_thread_flag(tsk, TIF_DEBUG);
 
 	tsk->thread.debugreg0 = 0;
 	tsk->thread.debugreg1 = 0;