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Date: Mon, 11 Feb 2008 23:46:28 -0800 From: Andrew Morton <akpm@...ux-foundation.org> To: Jonathan Corbet <corbet@....net> Cc: linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org Subject: Re: [PATCH] Avoid buffer overflows in get_user_pages() On Mon, 11 Feb 2008 16:17:33 -0700 Jonathan Corbet <corbet@....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@....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) { ... } ? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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