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

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

[Mathieu Desnoyers]

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 

It applies on 2.6.20.

Signed-off-by: Rebecca Schultz <rschultz@google.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>

--- a/arch/x86_64/kernel/process.c
+++ b/arch/x86_64/kernel/process.c
@@ -385,14 +385,17 @@ 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;
-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68

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

[David Miller]

From: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Date: Thu, 8 Mar 2007 21:15:53 -0500

> 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 
> 
> It applies on 2.6.20.
> 
> Signed-off-by: Rebecca Schultz <rschultz@google.com>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>

Please fix all platforms, not just your favorite one, kthx.
:-)

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

[Mathieu Desnoyers]

* David Miller (davem@davemloft.net) wrote:
> From: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
> Date: Thu, 8 Mar 2007 21:15:53 -0500
> 
> > 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 
> > 
> > It applies on 2.6.20.
> > 
> > Signed-off-by: Rebecca Schultz <rschultz@google.com>
> > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
> 
> Please fix all platforms, not just your favorite one, kthx.
> :-)

Please be comprehensive of the speed at which fingers can type on my
keyboard. Your quickness at replying to email is impressive though ;)

I am working on it, it won't take long.

Regards,

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68