Subject: Re: [PATCH] crypto: nx: Fix memcpy() over-reading in nonce

From: Kees Cook <[email-masked]>
Date: Fri, 18 Jun 2021 16:49:34 -0700

On Thu, Jun 17, 2021 at 04:08:15PM +1000, Michael Ellerman wrote:
> Kees Cook <[email-masked]> writes:
> > Fix typo in memcpy() where size should be CTR_RFC3686_NONCE_SIZE.
> >
> > Fixes: 030f4e968741 ("crypto: nx - Fix reentrancy bugs")
> > Cc: [email-masked]
> > Signed-off-by: Kees Cook <[email-masked]>
> 
> Thanks.
> 
> > ---
> >  drivers/crypto/nx/nx-aes-ctr.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/crypto/nx/nx-aes-ctr.c b/drivers/crypto/nx/nx-aes-ctr.c
> > index 13f518802343..6120e350ff71 100644
> > --- a/drivers/crypto/nx/nx-aes-ctr.c
> > +++ b/drivers/crypto/nx/nx-aes-ctr.c
> > @@ -118,7 +118,7 @@ static int ctr3686_aes_nx_crypt(struct skcipher_request *req)
> >  	struct nx_crypto_ctx *nx_ctx = crypto_skcipher_ctx(tfm);
> >  	u8 iv[16];
> >  
> > -	memcpy(iv, nx_ctx->priv.ctr.nonce, CTR_RFC3686_IV_SIZE);
> > +	memcpy(iv, nx_ctx->priv.ctr.nonce, CTR_RFC3686_NONCE_SIZE);
> >  	memcpy(iv + CTR_RFC3686_NONCE_SIZE, req->iv, CTR_RFC3686_IV_SIZE);
> >  	iv[12] = iv[13] = iv[14] = 0;
> >  	iv[15] = 1;
> 
> Where IV_SIZE is 8 and NONCE_SIZE is 4.
> 
> And iv is 16 bytes, so it's not a buffer overflow.
> 
> But priv.ctr.nonce is 4 bytes, and at the end of the struct, so it reads
> 4 bytes past the end of the nx_crypto_ctx, which is not good.
> 
> But then immediately overwrites whatever it read with req->iv.
> 
> So seems pretty harmless in practice?

Right -- there's no damage done, but future memcpy() FORTIFY work alerts
on this, so I'm going through cleaning all of these up. :)

-- 
Kees Cook