linux-kernel - Re: [PATCH] exec: fix set_binfmt() vs sys_delete

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Date:	Tue, 28 Jul 2009 16:58:17 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Amerigo Wang <xiyou.wangcong@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>,
	Roland McGrath <roland@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] exec: fix set_binfmt() vs sys_delete_module() race

On 07/28, Amerigo Wang wrote:
>
> On Fri, Jul 24, 2009 at 07:19:43PM +0200, Oleg Nesterov wrote:
> >sys_delete_module() can set MODULE_STATE_GOING after search_binary_handler()
> >does try_module_get(). In this case set_binfmt()->try_module_get() fails but
> >since none of the callers check the returned error, the task will run with
> >the wrong old ->binfmt.
> >
> >The proper fix should change all ->load_binary() methods, but we can rely
> >on fact that the caller must hold a reference to binfmt->module and use
> >__module_get() which never fails.
> >
>
> Sounds reasonable.
>
> Would like to put the last words as comments into code below?

Yes, thanks.

Rusty pointed out this too, and I already sent the updated patch.
But due to my mistake (I forgot to CC lkml) this was discussed
off-list.

> >-int set_binfmt(struct linux_binfmt *new)
> >+void set_binfmt(struct linux_binfmt *new)
> > {
> >-	struct linux_binfmt *old = current->binfmt;
> >+	if (current->binfmt)
> >+		module_put(current->binfmt->module);
> >
> >-	if (new) {
> >-		if (!try_module_get(new->module))
> >-			return -1;
> >-	}
> > 	current->binfmt = new;
> >-	if (old)
> >-		module_put(old->module);
> >-	return 0;
> >+	if (new)
> >+		__module_get(new->module);
>
>
> I prefer to put the 'current->binfmt = new;' line as the last
> statement within this function, since this is more readable for me.

Perhaps... but this is purely cosmetic, and the patch is already
in -mm. Unless you have a strong feeleing, I'd prefer to not send
yet another update.

Oleg.

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