[PATCH v2] execve: must clear current->clear_child_tid

[Oleg Nesterov <oleg@redhat.com>]

On 07/31, Andrew Morton wrote:
>
> On Fri, 31 Jul 2009 17:38:14 -0700 (PDT) Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
> >
> >
> > On Sat, 1 Aug 2009, Oleg Nesterov wrote:
> > >
> > > Perhaps it is better to change mm_release() ? It has to play with
> > > ->clear_child_tid anyway.
> >
> > Ahh. I take back my previous Ack. Your patch is better. I'll ack that
> > instead.
> >
>
> 'k, thanks.  I shall compulsively watch my inbox awaiting the signed-off
> and tested version ;)

I did some testing, but didn't try to check if this patches fixes the
origianal problem. It obviously should... Still I removed Tested-by tag.
But added Linus's ack, the patch is the same.

------------------------------------------------------------------------------
[PATCH v2] execve: must clear current->clear_child_tid

From: Eric Dumazet <eric.dumazet@gmail.com>

While looking at Jens Rosenboom bug report
(http://lkml.org/lkml/2009/7/27/35) about strange sys_futex call done from
a dying "ps" program, we found following problem.

clone() syscall has special support for TID of created threads.  This
support includes two features.

One (CLONE_CHILD_SETTID) is to set an integer into user memory with the
TID value.

One (CLONE_CHILD_CLEARTID) is to clear this same integer once the created
thread dies.

The integer location is a user provided pointer, provided at clone()
time.

kernel keeps this pointer value into current->clear_child_tid.

At execve() time, we should make sure kernel doesnt keep this user
provided pointer, as full user memory is replaced by a new one.

As glibc fork() actually uses clone() syscall with CLONE_CHILD_SETTID and
CLONE_CHILD_CLEARTID set, chances are high that we might corrupt user
memory in forked processes.

Following sequence could happen:

1) bash (or any program) starts a new process, by a fork() call that
   glibc maps to a clone( ...  CLONE_CHILD_SETTID | CLONE_CHILD_CLEARTID
   ...) syscall

2) When new process starts, its current->clear_child_tid is set to a
   location that has a meaning only in bash (or initial program) context
   (&THREAD_SELF->tid)

3) This new process does the execve() syscall to start a new program. 
   current->clear_child_tid is left unchanged (a non NULL value)

4) If this new program creates some threads, and initial thread exits,
   kernel will attempt to clear the integer pointed by
   current->clear_child_tid from mm_release() :

        if (tsk->clear_child_tid
            && !(tsk->flags & PF_SIGNALED)
            && atomic_read(&mm->mm_users) > 1) {
                u32 __user * tidptr = tsk->clear_child_tid;
                tsk->clear_child_tid = NULL;

                /*
                 * We don't check the error code - if userspace has
                 * not set up a proper pointer then tough luck.
                 */
<< here >>      put_user(0, tidptr);
                sys_futex(tidptr, FUTEX_WAKE, 1, NULL, NULL, 0);
        }

5) OR : if new program is not multi-threaded, but spied by /proc/pid
   users (ps command for example), mm_users > 1, and the exiting program
   could corrupt 4 bytes in a persistent memory area (shm or memory mapped
   file)

If current->clear_child_tid points to a writeable portion of memory of the
new program, kernel happily and silently corrupts 4 bytes of memory, with
unexpected effects.

Fix is straightforward and should not break any sane program.

Reported-by: Jens Rosenboom <jens@mcbone.net>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---

 kernel/fork.c |   22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

--- WAIT/kernel/fork.c~CLEARTID	2009-07-02 19:27:36.000000000 +0200
+++ WAIT/kernel/fork.c	2009-08-01 03:36:59.000000000 +0200
@@ -568,18 +568,18 @@ void mm_release(struct task_struct *tsk,
 	 * the value intact in a core dump, and to save the unnecessary
 	 * trouble otherwise.  Userland only wants this done for a sys_exit.
 	 */
-	if (tsk->clear_child_tid
-	    && !(tsk->flags & PF_SIGNALED)
-	    && atomic_read(&mm->mm_users) > 1) {
-		u32 __user * tidptr = tsk->clear_child_tid;
+	if (tsk->clear_child_tid) {
+		if (!(tsk->flags & PF_SIGNALED) &&
+		    atomic_read(&mm->mm_users) > 1) {
+			/*
+			 * We don't check the error code - if userspace has
+			 * not set up a proper pointer then tough luck.
+			 */
+			put_user(0, tsk->clear_child_tid);
+			sys_futex(tsk->clear_child_tid, FUTEX_WAKE,
+					1, NULL, NULL, 0);
+		}
 		tsk->clear_child_tid = NULL;
-
-		/*
-		 * We don't check the error code - if userspace has
-		 * not set up a proper pointer then tough luck.
-		 */
-		put_user(0, tidptr);
-		sys_futex(tidptr, FUTEX_WAKE, 1, NULL, NULL, 0);
 	}
 }