linux-kernel - Re: [PATCH 7/8] crypto: AF_ALG: add random number generator support

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Date:	Wed, 12 Nov 2014 18:46:44 +0100
From:	Stephan Mueller <smueller@...onox.de>
To:	Daniel Borkmann <dborkman@...hat.com>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	ABI/API <linux-api@...r.kernel.org>,
	linux-crypto@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 7/8] crypto: AF_ALG: add random number generator support

Am Mittwoch, 12. November 2014, 18:23:27 schrieb Daniel Borkmann:

Hi Daniel,

>On 11/12/2014 05:54 PM, Stephan Mueller wrote:
>> Am Mittwoch, 12. November 2014, 17:15:52 schrieb Daniel Borkmann:
>>> On 11/12/2014 08:05 AM, Stephan Mueller wrote:
>>>> This patch adds the random number generator support for AF_ALG.
>>>> 
>>>> A random number generator's purpose is to generate data without
>>>> requiring the caller to provide any data. Therefore, the AF_ALG
>>>> interface handler for RNGs only implements a callback handler for
>>>> recvmsg.
>>> 
>>> ...
>>> 
>>>> +static int rng_recvmsg(struct kiocb *unused, struct socket *sock,
>>>> +		       struct msghdr *msg, size_t len, int flags)
>>>> +{
>>>> +	struct sock *sk = sock->sk;
>>>> +	struct alg_sock *ask = alg_sk(sk);
>>>> +	struct rng_ctx *ctx = ask->private;
>>>> +	int err = -EFAULT;
>>>> +
>>>> +	if (0 == len)
>>> 
>>> if (len == 0)
>>> 
>>> 	...
>>> 
>>> [And also other places.]
>>> 
>>> We don't use Yoda condition style in the kernel.
>> 
>> Well, there is a very good reason for using the approach I have: we
>> all have done the error of forgetting the second = sign.
>> 
>> In my case, the compiler will complain and we fix the error right
>> away.
>> 
>> In your case, nobody is complaining but we introduced a nasty,
>> potentially hard to debug error. Thus, I very much like to keep my
>> version just to be on the safe side.
>> 
>> Note, there was even a backdoor I have seen where the missing 2nd
>> equal sign introduced a privilege escalation.
>> 
>> Therefore, my standard coding practice is to have a fixed value on
>> the left side and the variable on the right side of any comparison.
>
>I understand, but then please add this proposal first into ...
>
>   Documentation/CodingStyle
>
>The problem is that while the rest of the kernel does not follow
>this coding style, it's also much harder to read and/or program
>this way for people not being used to. So the danger of bugs
>slipping in this way is at least equally high. Besides that, this
>argument would also only account for '==' checks.

Ok, I can change that throughout the code.
>
>>>> +		return 0;
>>>> +	if (MAXSIZE < len)
>>>> +		len = MAXSIZE;
>>>> +
>>>> +	lock_sock(sk);
>>>> +	len = crypto_rng_get_bytes(ctx->drng, ctx->result, len);
>>>> +	if (0 > len)
>>>> +		goto unlock;
>>>> +
>>>> +	err = memcpy_toiovec(msg->msg_iov, ctx->result, len);
>>>> +	memset(ctx->result, 0, err);
>>>> +
>>> 
>>> This looks buggy.
>>> 
>>> If copy_to_user() fails from within memcpy_toiovec(), we call
>>> memset()
>>> with a negative return value which is interpreted as size_t and thus
>>> causes a buffer overflow writing beyond ctx->result, no?
>>> 
>>> If it succeeds, we call memset(ctx->result, 0, 0) .....
>> 
>> Right, good catch, I have to add a catch for negative error here.
>
>Hm? Don't you rather mean to say to unconditionally do something like
>...
>
>   memzero_explicit(ctx->result, len);

Sorry, I was not clear:

* I need to catch a failing memcpy, but not return an error.

* I unconditionally use the memset after memcpy as you indicated. Once 
the cryptodev tree contains the memzero_explicit call, I will start 
picking up that function.

Essentially, I throught of the line you suggested.

Ciao
Stephan
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