linux-kernel - Re: [Patch] mm: fix null pointer dereference in vm_normal

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Date:	Wed, 18 Feb 2009 18:23:37 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	Américo Wang <xiyou.wangcong@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
	Andrew Morton <akpm@...l.org>,
	Nick Piggin <nickpiggin@...oo.com.au>
Subject: Re: [Patch] mm: fix null pointer dereference in vm_normal_page()

On Wed, Feb 18, 2009 at 08:56:49PM +0800, Américo Wang wrote:
> 
> One usage of vm_normal_page() is:
> 
>     struct page *page = vm_normal_page(gate_vma, start, *pte);
> 
> where gate_vma is returned by get_gate_vma() which can be NULL.
> So let vm_normal_page return NULL when vma is NULL.

I assume you refer to __get_user_pages()...?

This function checks whether the address is in the gate area and only
iff so requests the VMA representing it.

If you really did see an oops that is worked-around by your patch,
then the in_gate_area()/get_gate_vma() in question are broken.

> Signed-off-by: WANG Cong <wangcong@...ux.org>
> Cc: Nick Piggin <nickpiggin@...oo.com.au>

  Nacked-by: Johannes Weiner <hannes@...xchg.org>

> ---
> diff --git a/mm/memory.c b/mm/memory.c
> index baa999e..e428aa6 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -493,6 +493,9 @@ struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
>  {
>  	unsigned long pfn = pte_pfn(pte);
>  
> +	if (!vma)
> +		return NULL;
> +
>  	if (HAVE_PTE_SPECIAL) {
>  		if (likely(!pte_special(pte)))
>  			goto check_pfn;
> 
> 
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