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From: Andrew Morton <akpm@linux-foundation.org>
To: Jonathan Corbet <corbet@lwn.net>
Cc: linux-kernel@vger.kernel.org, torvalds@linux-foundation.org
Subject: Re: [PATCH] Avoid buffer overflows in get_user_pages()
Date: Mon, 11 Feb 2008 23:46:28 -0800	[thread overview]
Message-ID: <20080211234628.b5533b87.akpm@linux-foundation.org> (raw)
In-Reply-To: <5758.1202771853@vena.lwn.net>

On Mon, 11 Feb 2008 16:17:33 -0700 Jonathan Corbet <corbet@lwn.net> wrote:

> Avoid buffer overflows in get_user_pages()
> 
> So I spent a while pounding my head against my monitor trying to figure
> out the vmsplice() vulnerability - how could a failure to check for
> *read* access turn into a root exploit?  It turns out that it's a buffer
> overflow problem which is made easy by the way get_user_pages() is
> coded.
> 
> In particular, "len" is a signed int, and it is only checked at the
> *end* of a do {} while() loop.  So, if it is passed in as zero, the loop
> will execute once and decrement len to -1.  At that point, the loop will
> proceed until the next invalid address is found; in the process, it will
> likely overflow the pages array passed in to get_user_pages().

Sounds convincing.

> I think that, if get_user_pages() has been asked to grab zero pages,
> that's what it should do.  Thus this patch; it is, among other things,
> enough to block the (already fixed) root exploit and any others which
> might be lurking in similar code.  I also think that the number of pages
> should be unsigned, but changing the prototype of this function probably
> requires some more careful review.
> 
> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index e5628a5..7f50fd8 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -989,6 +989,8 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
>  	int i;
>  	unsigned int vm_flags;
>  
> +	if (len <= 0)
> +		return 0;
>  	/* 
>  	 * Require read or write permissions.
>  	 * If 'force' is set, we only require the "MAY" flags.

Can we just convert

	do {
		...
	} while (len);

into

	while (len) {
		...
	}

?

  parent reply	other threads:[~2008-02-12  7:47 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-02-11 23:17 Jonathan Corbet
2008-02-11 23:45 ` Nick Piggin
2008-02-12  7:46 ` Andrew Morton [this message]
2008-02-12 10:35   ` Jiri Kosina
2008-02-14 16:45 ` Oliver Pinter
2008-02-14 21:09   ` Jonathan Corbet
     [not found] <fa.5jOFf0zR7ZoK3hLDItf3Omow4lE@ifi.uio.no>
     [not found] ` <fa.NNs+hqAlLlf93+yNZ/YJzSyGQbs@ifi.uio.no>
2008-02-12  3:16   ` Robert Hancock
2008-02-12  5:56     ` Nick Piggin
     [not found] <9VQ6w-5Xn-7@gated-at.bofh.it>
     [not found] ` <9VY4a-1tI-21@gated-at.bofh.it>
2008-02-12  8:34   ` Bodo Eggert

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