[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: Rebecca Schultz <rschultz@google.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Acked-by: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 5 deletions

diff --git a/arch/x86_64/kernel/process.c b/arch/x86_64/kernel/process.c
index cbbc6adc1a92..d8d5ccc245c8 100644
--- a/arch/x86_64/kernel/process.c
+++ b/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;