linux-kernel - Re: [PATCH] sysctl: terminate strings also on \r

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Date:	Thu, 23 Oct 2014 09:39:09 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Paul Wise <pabs3@...edaddy.net>,
	Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Aaron Tomlin <atomlin@...hat.com>,
	Dario Faggioli <raistlin@...ux.it>,
	Andi Kleen <ak@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Jens Axboe <axboe@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] sysctl: terminate strings also on \r

On Wed, Oct 22, 2014 at 7:00 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Wed, 22 Oct 2014 16:43:10 -0700 Kees Cook <keescook@...omium.org> wrote:
>
>> On Wed, Oct 22, 2014 at 4:26 PM, Andrew Morton
>> <akpm@...ux-foundation.org> wrote:
>> > On Tue, 21 Oct 2014 13:21:37 -0700 Kees Cook <keescook@...omium.org> wrote:
>> >
>> >> From: Paul Wise <pabs3@...edaddy.net>
>> >>
>> >> This partially mitigates a common strategy used by attackers for hiding
>> >> the full contents of strings in procfs from naive sysadmins who use cat,
>> >> more or sysctl to inspect the contents of strings in procfs.
>> >>
>> >> ...
>> >>
>> >> --- a/kernel/sysctl.c
>> >> +++ b/kernel/sysctl.c
>> >> @@ -1739,7 +1739,7 @@ static int _proc_do_string(char *data, int maxlen, int write,
>> >>               while ((p - buffer) < *lenp && len < maxlen - 1) {
>> >>                       if (get_user(c, p++))
>> >>                               return -EFAULT;
>> >> -                     if (c == 0 || c == '\n')
>> >> +                     if (c == 0 || c == '\n' || c == '\r')
>> >>                               break;
>> >>                       data[len++] = c;
>> >>               }
>> >
>> > There are no valid uses of \r in a procfs write?
>>
>> I struggle to imagine one; everything I found that uses proc_dostring
>> seems to be names, paths, and commands.
>>
>
> You're insufficiently pessimistic.

Haha, I haven't had that accusation made about me before; I'll keep
this quote around! :)

> I wonder if the chances of damage would be lower if we were to continue
> to accept the \r, but turn it into something else ("\r"?) when it is
> read.

I think that would complicate things more than help them. If there's a
legit use of \r, I'll let Paul Wise debate how to proceed. :)

-Kees

-- 
Kees Cook
Chrome OS Security
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