Re: buffer overflow in /proc/sys/sunrpc/transports

[Vegard Nossum]

On Sat, Aug 30, 2008 at 9:06 PM, Cyrill Gorcunov <gorcunov@xxxxxxxxx> 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;
>
>

BTW, look at this:

    $ od -A x -t x1z /proc/sys/sunrpc/transports
    000000 74 63 70 20 31 30 34 38 35 37 36 0a 75 64 70 20  >tcp 1048576.udp <
    000010 33 32 37 36 38 0a 00 00 00 00 00 00 00 00 00 00  >32768...........<
    000020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  >................<
    *
    0003e0 00 00 00 00 00 00 00 00 00 00                    >..........<
    0003ea

...and:

    $ strace -e trace=read cat /proc/sys/sunrpc/transports > /dev/null
    read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0@G\316E4\0\0\0"...,
512) = 512
    read(3, "tcp 1048576\nudp 32768\n\0\0\0\0\0\0\0\0\0\0"..., 4096) = 4074
    read(3, "", 4096)                       = 0

...why does it have a huge return value? The output is only about 40
bytes... why add all the \0? Would your patch also fix this?


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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