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

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Message-ID: <20080831103026.GF7391@lenovo>
Date:	Sun, 31 Aug 2008 14:30:26 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	David Wagner <daw-news@...berkeley.edu>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: buffer overflow in /proc/sys/sunrpc/transports

[David Wagner - Sat, Aug 30, 2008 at 10:55:51PM +0000]
| Cyrill Gorcunov  wrote:
| >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;
| 
| 1. Would it be better to use copy_to_user() rather than
| access_ok() followed immediately by __copy_to_user()?

Yes, thanks

| 
| 2. Is it OK to dereference *lenp directly?  Is lenp a pointer into user
| memory or kernel memory?  If it points to user memory, why is it safe to
| dereference it directly?  (What about TOCTTOU bugs?)  Should there be
| some sparse annotations here to ensure the code is not dereferencing
| user pointers directly?  Later on, proc_do_xprt() also dereferences
| *lenp and *ppos directly.

Not only proc_do_xprt do that so I think it's safe (check for NULL
on highr level I suspect).

| 
| 3. 'len' is declared as a signed int.  len will be converted to size_t
| before doing the comparison, so if len can ever be negative (e.g.,
| svc_print_xprts() returns -1 because of an error), this patch will do
| the wrong thing.  Looks like the current definition of svc_print_xprts()
| won't ever do that, as that code currently stands, so at present this
| is not a bug.  However from a security point of view there are benefits
| to code whose correctness is 'locally obvious', all else being equal.
| In particular this seems like a possible maintenance hazard.  Would it be
| better to use type size_t for lengths like this that are never supposed
| to be negative?

thanks, I changed it to size_t and as you mentoined negative is never coming
from called svc_print_xprts.

| 
| 4. Is proc_dostring() relevant here?

it's not possible to use for now this function (one of my patches
was quite wrong by using it :)

		- Cyrill -
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