linux-kernel - [PATCH] Avoid buffer overflows in get_user

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Message-ID: <5758.1202771853@vena.lwn.net>
Date:	Mon, 11 Feb 2008 16:17:33 -0700
From:	Jonathan Corbet <corbet@....net>
To:	linux-kernel@...r.kernel.org
cc:	akpm@...ux-foundation.org, torvalds@...ux-foundation.org
Subject: [PATCH] Avoid buffer overflows in get_user_pages()

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@....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.
--
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