[2.6.22.y #2] Be more robust about bad arguments in get_user_pages()

[Oliver Pinter]

 From a8a7690626756b6dcd49ad23b58f4406bfa59d7f Mon Sep 17 00:00:00 2001
 From: Jonathan Corbet <corbet@xxxxxxx>
 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@xxxxxxx>
 Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
 CC: Oliver Pinter <oliver.pntr@xxxxxxxxx>
 
 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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