Re: [PATCH] Avoid buffer overflows in get_user_pages()
- From: "Oliver Pinter" <oliver.pntr@xxxxxxxxx>
- Date: Thu, 14 Feb 2008 17:45:51 +0100
for stable (.22 .23 .24) ?
git id in mainline: 900cf086fd2fbad07f72f4575449e0d0958f860f
--
tested in: http://repo.or.cz/w/linux-2.6.22.y-op.git testing
On 2/12/08, Jonathan Corbet <corbet@xxxxxxx> 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;
/*
* Require read or write permissions.
* If 'force' is set, we only require the "MAY" flags.
--
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Thanks,
Oliver
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