linux-kernel - Re: [PATCH 2/2] dh key: get rid of stack array allocation

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Message-ID: <20180315022112.GB641@zzz.localdomain>
Date:   Wed, 14 Mar 2018 19:21:12 -0700
From:   Eric Biggers <ebiggers3@...il.com>
To:     Tycho Andersen <tycho@...ho.ws>
Cc:     David Howells <dhowells@...hat.com>, keyrings@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>
Subject: Re: [PATCH 2/2] dh key: get rid of stack array allocation

On Mon, Mar 12, 2018 at 10:29:07PM -0600, Tycho Andersen wrote:
> Similarly to the previous patch, we would like to get rid of stack
> allocated arrays: https://lkml.org/lkml/2018/3/7/621
> 
> In this case, we can also use a malloc style approach to free the temporary
> buffer, being careful to also use kzfree to free them (indeed, at least one
> of these has a memzero_explicit, but it seems like maybe they both
> should?).
> 
> Signed-off-by: Tycho Andersen <tycho@...ho.ws>
> CC: David Howells <dhowells@...hat.com>
> CC: James Morris <jmorris@...ei.org>
> CC: "Serge E. Hallyn" <serge@...lyn.com>
> ---
>  security/keys/dh.c | 27 +++++++++++++++++++++------
>  1 file changed, 21 insertions(+), 6 deletions(-)
> 
> diff --git a/security/keys/dh.c b/security/keys/dh.c
> index d1ea9f325f94..f02261b24759 100644
> --- a/security/keys/dh.c
> +++ b/security/keys/dh.c
> @@ -162,19 +162,27 @@ static int kdf_ctr(struct kdf_sdesc *sdesc, const u8 *src, unsigned int slen,
>  			goto err;
>  
>  		if (zlen && h) {
> -			u8 tmpbuffer[h];
> +			u8 *tmpbuffer;
>  			size_t chunk = min_t(size_t, zlen, h);
> -			memset(tmpbuffer, 0, chunk);
> +
> +			err = -ENOMEM;
> +			tmpbuffer = kzalloc(chunk, GFP_KERNEL);
> +			if (!tmpbuffer)
> +				goto err;
>  
>  			do {
>  				err = crypto_shash_update(desc, tmpbuffer,
>  							  chunk);
> -				if (err)
> +				if (err) {
> +					kzfree(tmpbuffer);
>  					goto err;
> +				}
>  
>  				zlen -= chunk;
>  				chunk = min_t(size_t, zlen, h);
>  			} while (zlen);
> +
> +			kzfree(tmpbuffer);
>  		}

This is just hashing zeroes.  Why not use the zeroes at the end of the 'src'
buffer which was allocated as 'outbuf' in __keyctl_dh_compute()?  It's already
the right size.  It might even simplify the code a bit since
crypto_shash_update() would no longer need to be in a loop.

>  
>  		if (src && slen) {
> @@ -184,13 +192,20 @@ static int kdf_ctr(struct kdf_sdesc *sdesc, const u8 *src, unsigned int slen,
>  		}
>  
>  		if (dlen < h) {
> -			u8 tmpbuffer[h];
> +			u8 *tmpbuffer;
> +
> +			err = -ENOMEM;
> +			tmpbuffer = kzalloc(h, GFP_KERNEL);
> +			if (!tmpbuffer)
> +				goto err;
>  
>  			err = crypto_shash_final(desc, tmpbuffer);
> -			if (err)
> +			if (err) {
> +				kzfree(tmpbuffer);
>  				goto err;
> +			}
>  			memcpy(dst, tmpbuffer, dlen);
> -			memzero_explicit(tmpbuffer, h);
> +			kzfree(tmpbuffer);
>  			return 0;
>  		} else {
>  			err = crypto_shash_final(desc, dst);
> -- 

Why not instead round the allocated size of 'outbuf' in keyctl_dh_compute_kdf()
up to the next 'crypto_shash_digestsize()'-boundary?  Then this temporary buffer
wouldn't be needed at all.

It would be nice if people thought about how to properly solve the problems when
doing these VLA conversions, rather than mindlessly replacing them with
kmalloc...

Eric