linux-kernel - [2.6.22.y #2] Be more robust about bad arguments in get_user

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Date:	Mon, 18 Feb 2008 21:01:20 +0100
From:	"Oliver Pinter" <oliver.pntr@...il.com>
To:	"Linux Kernel" <linux-kernel@...r.kernel.org>, stable@...nel.org,
	stable-commits@...r.kernel.org
Cc:	"Jonathan Corbet" <corbet@....net>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Oliver Pinter" <oliver.pntr@...il.com>,
	"Adrian Bunk" <bunk@...nel.org>, "Greg KH" <gregkh@...e.de>,
	"Justin Forbes" <jmforbes@...uxtx.org>,
	"Zwane Mwaikambo" <zwane@....linux.org.uk>,
	"Theodore Ts'o" <tytso@....edu>,
	"Randy. Dunlap" <rdunlap@...otime.net>,
	"Chuck Wolber" <chuckw@...ntumlinux.com>,
	"Dave Jones" <davej@...hat.com>,
	"Chris Wedgwood" <reviews@...cw.f00f.org>,
	"Michael Krufky" <mkrufky@...uxtv.org>,
	"Chuck Ebbert" <cebbert@...hat.com>,
	"Domenico Andreoli" <cavokz@...il.com>,
	"chrisw@...s-sol.org" <chrisw@...s-sol.org>
Subject: [2.6.22.y #2] Be more robust about bad arguments in get_user_pages()

 From a8a7690626756b6dcd49ad23b58f4406bfa59d7f Mon Sep 17 00:00:00 2001
 From: Jonathan Corbet <corbet@....net>
 Date: Mon, 11 Feb 2008 16:17:33 -0700
 Subject: [PATCH] Be more robust about bad arguments in get_user_pages()

 MAINLINE: 900cf086fd2fbad07f72f4575449e0d0958f860f

 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().

 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@....net>
 Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
 CC: Oliver Pinter <oliver.pntr@...il.com>

 diff --git a/mm/memory.c b/mm/memory.c
 index f64cbf9..538f054 100644
 --- a/mm/memory.c
 +++ b/mm/memory.c
 @@ -983,6 +983,8 @@ 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.

 -- 
 Thanks,
 Oliver

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