Re: fuzz tested user mode linux core dumps in fs/lockd/clntproc.c:131

[Oleg Nesterov <oleg@redhat.com>]

On 07/27, Toralf Förster wrote:
>
> I do have a user mode linux image (stable 32 bit Gentoo Linux ) which erratically crashes
> while fuzz tested with trinity if the victim files are located on a NFS share.
>
> The back trace of the core dumps always looks like the attached.
>
> To bisect it is hard. However after few attempts in the last weeks the following
> commit is either the first bad commit or at least the upper limit (less likely).
>
>
> commit 8aac62706adaaf0fab02c4327761561c8bda9448
> Author: Oleg Nesterov <oleg@redhat.com>
> Date:   Fri Jun 14 21:09:49 2013 +0200
>
>     move exit_task_namespaces() outside of exit_notify()
>
> #15 nlmclnt_setlockargs (req=0x48e18860, fl=0x48f27c8c) at fs/lockd/clntproc.c:131

Thanks.

So nlmclnt_setlockargs()->utsname() crashes and we probably need
the patch below.

But is it correct? I know _absolutely_ nothing about nfs/sunrpc/etc and
I never looked into this code before, most probably I am wrong.

But it seems that __nlm_async_call() relies on workqueues.
nlmclnt_async_call() does rpc_wait_for_completion_task(), but what if
the caller is killed?

nlm_rqst can't go away, ->a_count was incremented. But can't the caller
exit before call->name is used? In this case the memory it points to
can be already freed.

Oleg.

--- x/kernel/exit.c
+++ x/kernel/exit.c
@@ -783,8 +783,8 @@ void do_exit(long code)
 	exit_shm(tsk);
 	exit_files(tsk);
 	exit_fs(tsk);
-	exit_task_namespaces(tsk);
 	exit_task_work(tsk);
+	exit_task_namespaces(tsk);
 	check_stack_usage();
 	exit_thread();