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

Nick Piggin
2008-02-12T05:56:35Z

On Tuesday 12 February 2008 14:16, Robert Hancock wrote:
> 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@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;
> >
> > BUG_ON()?
>
> Well, not if the code involved in the exploit can pass a zero value,

Which is a bug, and you want to catch it.

> otherwise it's just turning it into a DoS..

If it is due to a security bug, then the fix is to fix the point
where the kernel starts trusting an untrusted value. Not to hide
the bug like this. Arguably, a BUG_ON is better in the case of a
security hole because you want to halt the process as soon as you
detect a problem.