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

[Cyrill Gorcunov <gorcunov@gmail.com>]

[Cyrill Gorcunov - Sat, Aug 30, 2008 at 11:21:12PM +0400]
| [Vegard Nossum - Sat, Aug 30, 2008 at 09:15:16PM +0200]
| | On Sat, Aug 30, 2008 at 9:06 PM, Cyrill Gorcunov <gorcunov@gmail.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?
| 
| Well, hard to say what user-space programmer is expecting from us.
| I mean - maybe he (reader) wants only part of results not the whole
| contents BUT by this way he never know what the whole conetnts would be
| until trying to read more (ie to check if there no more data from
| kernel side). What is preferred behaviour - i don't know :)
| 
| | 
| | 
| | 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
| | 
| 
| 		- Cyrill -

Btw, I didn't try to get size of sysfs file - will such an action return
size of data from kernel side?

		- Cyrill -