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

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Date:	Sat, 26 Jul 2014 19:35:22 -0700
From:	Mark D Rustad <mrustad@...il.com>
To:	Rickard Strandqvist <rickard_strandqvist@...ctrumdigital.se>
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
Subject: Re: [PATCH] crypto: rng.c:  Cleaning up missing null-terminate in conjunction with strncpy

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.

-- 
Mark Rustad, MRustad@...il.com

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