linux-kernel - Re: buffer overflow in /proc/sys/sunrpc/transports

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Message-ID: <19f34abd0808301215i445e8411q987c864c0b478d30@mail.gmail.com>
Date:	Sat, 30 Aug 2008 21:15:16 +0200
From:	"Vegard Nossum" <vegard.nossum@...il.com>
To:	"Cyrill Gorcunov" <gorcunov@...il.com>
Cc:	"Tom Tucker" <tom@...ngridcomputing.com>,
	"Neil Brown" <neilb@...e.de>,
	"Chuck Lever" <chuck.lever@...cle.com>, "Greg Banks" <gnb@....com>,
	"J. Bruce Fields" <bfields@...i.umich.edu>,
	linux-kernel@...r.kernel.org
Subject: Re: buffer overflow in /proc/sys/sunrpc/transports

On Sat, Aug 30, 2008 at 9:06 PM, Cyrill Gorcunov <gorcunov@...il.com> wrote:
> [Vegard Nossum - Sat, Aug 30, 2008 at 08:44:22PM +0200]
> | Hi,
> |
> | I noticed that something weird is going on with /proc/sys/sunrpc/transports.
> | This file is generated in net/sunrpc/sysctl.c, function proc_do_xprt(). When
> | I "cat" this file, I get the expected output:
> |
> |     $ cat /proc/sys/sunrpc/transports
> |     tcp 1048576
> |     udp 32768
> |
> | But I think that it does not check the length of the buffer supplied by
> | userspace to read(). With my original program, I found that the stack was
> | being overwritten by the characters above, even when the length given to
> | read() was just 1. So I have created a test program, see it at the bottom of
> | this e-mail. Here is its output:
> |
> ...
>
> Indeed, maybe just add checking for user buffer length?
> As proc_dodebug() in this file are doing. I don't think
> the user would be happy with his stack burned :)
>
> Something like:
> ---
>
> Index: linux-2.6.git/net/sunrpc/sysctl.c
> ===================================================================
> --- linux-2.6.git.orig/net/sunrpc/sysctl.c      2008-07-20 11:40:14.000000000 +0400
> +++ linux-2.6.git/net/sunrpc/sysctl.c   2008-08-30 23:05:30.000000000 +0400
> @@ -69,6 +69,8 @@ static int proc_do_xprt(ctl_table *table
>                return -EINVAL;
>        else {
>                len = svc_print_xprts(tmpbuf, sizeof(tmpbuf));
> +               if (*lenp < len)
> +                       return -EFAULT;
>                if (!access_ok(VERIFY_WRITE, buffer, len))
>                        return -EFAULT;
>

Hm. I think this is wrong. Shouldn't we copy as many bytes as the user
indicated?


Vegard

-- 
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
	-- E. W. Dijkstra, EWD1036
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