linux-kernel - Re: [PATCH 1/2] task_struct cleanup: move binfmt field to mm

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Message-ID: <20090728143734.GA6664@redhat.com>
Date:	Tue, 28 Jul 2009 16:37:34 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>
Cc:	Roland McGrath <roland@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 1/2] task_struct cleanup: move binfmt field to mm_struct

On 07/28, Hiroshi Shimamoto wrote:
>
> Oleg Nesterov wrote:
> > (add Rusty)
> >
> > On 07/27, Hiroshi Shimamoto wrote:
> >>  void set_binfmt(struct linux_binfmt *new)
> >>  {
> >> -	if (current->binfmt)
> >> -		module_put(current->binfmt->module);
> >> +	struct mm_struct *mm = current->mm;
> >> +
> >> +	BUG_ON(!mm);
> >> +	if (mm->binfmt)
> >
> > I am not sure we need this BUG_ON() above. If mm == NULL we will
> > have the same debug info after the crash.
>
> Ah, right. It will crash at accessing mm->binfmt and we'll get the
> same information. I didn't think seriously.
> BTW, now I noticed that it looks similar the security issue with
> NULL page mapping. If the kernel has the page mapping to NULL,
> it causes unstable behavior, no?

The kernel can't have NULL mapped. The task can, but in this case
it must have a valid ->mm != NULL.

IOW, if current->mm == NULL, current uses init_mm and thus NULL
can't be mapped.

Oleg.

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