Re: [RFC PATCH 1/2] crypto: Fix -Wstringop-truncation warnings

[Eric Biggers <ebiggers3@gmail.com>]

On Sat, Jun 23, 2018 at 11:07:52AM +0900, Stafford Horne wrote:
> As of GCC 9.0.0 the build is reporting warnings like:
> 
>     crypto/ablkcipher.c: In function ‘crypto_ablkcipher_report’:
>     crypto/ablkcipher.c:374:2: warning: ‘strncpy’ specified bound 64 equals destination size [-Wstringop-truncation]
>       strncpy(rblkcipher.geniv, alg->cra_ablkcipher.geniv ?: "<default>",
>       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>        sizeof(rblkcipher.geniv));
>        ~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> This means the strnycpy might create a non null terminated string.  Fix this by
> limiting the size of the string copy to include the null terminator.
> 
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Signed-off-by: Stafford Horne <shorne@gmail.com>
> ---
>  crypto/ablkcipher.c | 4 ++--
>  crypto/blkcipher.c  | 2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/crypto/ablkcipher.c b/crypto/ablkcipher.c
> index d880a4897159..972cd7c879f6 100644
> --- a/crypto/ablkcipher.c
> +++ b/crypto/ablkcipher.c
> @@ -372,7 +372,7 @@ static int crypto_ablkcipher_report(struct sk_buff *skb, struct crypto_alg *alg)
>  
>  	strncpy(rblkcipher.type, "ablkcipher", sizeof(rblkcipher.type));
>  	strncpy(rblkcipher.geniv, alg->cra_ablkcipher.geniv ?: "<default>",
> -		sizeof(rblkcipher.geniv));
> +		sizeof(rblkcipher.geniv) - 1);
>  
>  	rblkcipher.blocksize = alg->cra_blocksize;
>  	rblkcipher.min_keysize = alg->cra_ablkcipher.min_keysize;
> @@ -446,7 +446,7 @@ static int crypto_givcipher_report(struct sk_buff *skb, struct crypto_alg *alg)
>  
>  	strncpy(rblkcipher.type, "givcipher", sizeof(rblkcipher.type));
>  	strncpy(rblkcipher.geniv, alg->cra_ablkcipher.geniv ?: "<built-in>",
> -		sizeof(rblkcipher.geniv));
> +		sizeof(rblkcipher.geniv) - 1);
>  
>  	rblkcipher.blocksize = alg->cra_blocksize;
>  	rblkcipher.min_keysize = alg->cra_ablkcipher.min_keysize;
> diff --git a/crypto/blkcipher.c b/crypto/blkcipher.c
> index 01c0d4aa2563..f1644b5cf68c 100644
> --- a/crypto/blkcipher.c
> +++ b/crypto/blkcipher.c
> @@ -511,7 +511,7 @@ static int crypto_blkcipher_report(struct sk_buff *skb, struct crypto_alg *alg)
>  
>  	strncpy(rblkcipher.type, "blkcipher", sizeof(rblkcipher.type));
>  	strncpy(rblkcipher.geniv, alg->cra_blkcipher.geniv ?: "<default>",
> -		sizeof(rblkcipher.geniv));
> +		sizeof(rblkcipher.geniv) - 1);
>  
>  	rblkcipher.blocksize = alg->cra_blocksize;
>  	rblkcipher.min_keysize = alg->cra_blkcipher.min_keysize;

Your "fix" introduces an information disclosure bug, as it results in
uninitialized memory being copied to userspace.  This same broken patch was sent
by someone else too.

Maybe it would be best to just memset() the crypto_report_* structs to 0 after
declaration and then replace the strncpy()'s with strscpy()'s, even if just to
stop people from sending broken "fixes".  Do you want to do that?

- Eric