Re: [PATCH] kmod: Fix race in usermodehelper code

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 30 Jun 2009 22:48:03 -0400
Neil Horman <nhorman@tuxdriver.com> wrote:

> User Mode Helper: Fix race in UMH_WAIT_EXEC case
> 
> The user mode helper code has a race in it.  call_usermodehelper_exec takes an
> allocated subprocess_info structure, which it passes to a workqueue, and then
> passes it to a kernel thread which it creates, after which it calls complete to
> signal to the caller of call_usremodehelper_exec that it can free the
> subprocess_info struct.  But since we use that structure in the created thread,
> we can't call complete from __call_usermodehelper, which is where we create the
> kernel_thread.  We need to call complete from within the kernel thread and then
> not use subprocess_info afterward in the case of UMH_WAIT_EXEC.  Tested
> successfully by me.
> 

<stares at the code for ten minutes>

Geeze that's getting complex, isn't it?  The patch looks OK.  I think.

I wonder why people aren't reporting this.  Perhaps CLONE_VFORK plus
child-runs-first?  (_does_ the chld run first?  We seem to have changed
it a couple of times)