linux-kernel - Re: [PATCH] Avoid buffer overflows in get_user

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Message-Id: <E1JOqan-0000hX-1T@be1.7eggert.dyndns.org>
Date:	Tue, 12 Feb 2008 09:34:16 +0100
From:	Bodo Eggert <7eggert@....de>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH] Avoid buffer overflows in get_user_pages()

Andrew Morton <akpm@...ux-foundation.org> wrote:
> On Mon, 11 Feb 2008 16:17:33 -0700 Jonathan Corbet <corbet@....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().

[...]

> Can we just convert
> 
> do {
> ...
> } while (len);
> 
> into
> 
> while (len) {

while (len > 0), if I understand this patch correctly.

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