Messages in this thread | | | Date | Mon, 11 Feb 2008 23:46:28 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] Avoid buffer overflows in get_user_pages() |
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On Mon, 11 Feb 2008 16:17:33 -0700 Jonathan Corbet <corbet@lwn.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().
Sounds convincing.
> 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; > /* > * Require read or write permissions. > * If 'force' is set, we only require the "MAY" flags.
Can we just convert
do { ... } while (len);
into
while (len) { ... }
?
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