linux-kernel - Re: [PATCH] crypto: rng.c: Cleaning up missing null-terminate in conjunction with strncpy

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Date:	Sun, 27 Jul 2014 14:14:00 +0200
From:	Rickard Strandqvist <rickard_strandqvist@...ctrumdigital.se>
To:	Mark D Rustad <mrustad@...il.com>
Cc:	Neil Horman <nhorman@...driver.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>,
	linux-crypto@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] crypto: rng.c: Cleaning up missing null-terminate in
 conjunction with strncpy

2014-07-27 4:35 GMT+02:00 Mark D Rustad <mrustad@...il.com>:
> Rickard,
>
> On Jul 26, 2014, at 7:18 AM, Rickard Strandqvist <rickard_strandqvist@...ctrumdigital.se> wrote:
>
>> Replacing strncpy with strlcpy to avoid strings that lacks null terminate.
>>
>> Signed-off-by: Rickard Strandqvist <rickard_strandqvist@...ctrumdigital.se>
>> ---
>> crypto/rng.c |    2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/crypto/rng.c b/crypto/rng.c
>> index e0a25c2..c3d4fb3 100644
>> --- a/crypto/rng.c
>> +++ b/crypto/rng.c
>> @@ -65,7 +65,7 @@ static int crypto_rng_report(struct sk_buff *skb, struct crypto_alg *alg)
>> {
>>       struct crypto_report_rng rrng;
>>
>> -     strncpy(rrng.type, "rng", sizeof(rrng.type));
>> +     strlcpy(rrng.type, "rng", sizeof(rrng.type));
>>
>>       rrng.seedsize = alg->cra_rng.seedsize;
>
> Not to pick on this patch in particular, but you need to be careful about changing strncpy to strlcpy. Although strlcpy ensures termination, it does not prevent information leakage - strncpy ensures that the entire destination buffer is written. When leakage is a concern, it is better to use strncpy and then to store a zero in the last location of the buffer to ensure termination.
>
> These "simple" transformations can be risky - and many of these do not represent any sort of problem when the source is smaller than the destination. I hope information leakage is being considered.
>


Hi Mark

You clearly have a point there! Information leakage is not something I
think about very often otherwise.

It is really unbelievable that C messed up when trying to make a
better solution to strcpy. strncpy doing something completely new with
zeroes whole size but still does not guarantee a terminating null
character. And strlcpy that works great as a replacement for strcpy,
but really can not replace strncpy in all cases.  Incredibly stupid!
:-(

But ok, I'll submit a [Patch V2] with foo[sizeof(foo) - 1] = '\ 0'
on all under crypto/  then  ?


Kind regards
Rickard Strandqvist
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