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

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Date:   Sat, 23 Jun 2018 15:46:48 +0900
From:   Stafford Horne <shorne@...il.com>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Greg KH <gregkh@...uxfoundation.org>, arnd@...db.de,
        linux-crypto@...r.kernel.org,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Nick Desaulniers <nick.desaulniers@...il.com>
Subject: Re: [RFC PATCH 1/2] crypto: Fix -Wstringop-truncation warnings

On Sat, Jun 23, 2018 at 11:52:56AM +0900, Stafford Horne wrote:
> On Fri, Jun 22, 2018 at 07:41:49PM -0700, Eric Biggers wrote:
> > 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:
> > > 
[...]
> > >  	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?
> 
> Right, I didnt realize that we were using strncpy to also init the whole buffer.
> 
> I will do as suggest, and respin.

Hi Eric,

I thought about this a bit, doing memset() and strscpy() seemed fine, but the
below also would work, be a bit faster and stop gcc form complaining.  What do
you think?

@@ -512,6 +512,7 @@ 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));
+       rblkcipher.geniv[sizeof(rblkcipher.geniv) - 1] = '\0';

        rblkcipher.blocksize = alg->cra_blocksize;
        rblkcipher.min_keysize = alg->cra_blkcipher.min_keysize;

Let me know what you think.

-Stafford