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

[Oleg Nesterov <oleg@redhat.com>]

(Fix Serge's email)

On 07/27, Oleg Nesterov wrote:
>
> 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?

I meant lock->caller, sorry.

> In this case the memory it points to
> can be already freed.

And of course I have no idea what lock->caller actually means. But note
that the final fput() can be called by another process from the different
namespace. Say, a task from the parent namespace looks at /proc/pid/fd.

But again. I do not understand this code at all.

> 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();
>