Re: [PATCH] Avoid buffer overflows in get_user_pages()

[Robert Hancock]

Nick Piggin wrote:
> On Tuesday 12 February 2008 10:17, Jonathan Corbet 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().
> >
> > 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>
> >
> > 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;
>
> BUG_ON()?

Well, not if the code involved in the exploit can pass a zero value, 
otherwise it's just turning it into a DoS..
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