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Date: Tue, 24 Apr 2018 08:35:39 -0600
From: Tycho Andersen <tycho@...ho.ws>
To: Eric Biggers <ebiggers3@...il.com>
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>,
	"Jason A . Donenfeld" <Jason@...c4.com>
Subject: Re: [PATCH 1/3] big key: get rid of stack array allocation

Hi Eric,

On Mon, Apr 23, 2018 at 09:50:15PM -0700, Eric Biggers wrote:
> Hi Tycho,
> 
> On Mon, Apr 23, 2018 at 07:03:19PM -0600, Tycho Andersen wrote:
> > We're interested in getting rid of all of the stack allocated arrays in the
> > kernel [1]. This patch simply hardcodes the iv length to match that of the
> > hardcoded cipher.
> > 
> > [1]: https://lkml.org/lkml/2018/3/7/621
> > 
> > v2: hardcode the length of the nonce to be the GCM AES IV length, and do a
> >     sanity check in init(), Eric Biggers
> > 
> > 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>
> > CC: Jason A. Donenfeld <Jason@...c4.com>
> > CC: Eric Biggers <ebiggers3@...il.com>
> > ---
> >  security/keys/big_key.c | 9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> > 
> > diff --git a/security/keys/big_key.c b/security/keys/big_key.c
> > index 933623784ccd..75c46786a166 100644
> > --- a/security/keys/big_key.c
> > +++ b/security/keys/big_key.c
> > @@ -22,6 +22,7 @@
> >  #include <keys/user-type.h>
> >  #include <keys/big_key-type.h>
> >  #include <crypto/aead.h>
> > +#include <crypto/gcm.h>
> >  
> >  struct big_key_buf {
> >  	unsigned int		nr_pages;
> > @@ -109,7 +110,7 @@ static int big_key_crypt(enum big_key_op op, struct big_key_buf *buf, size_t dat
> >  	 * an .update function, so there's no chance we'll wind up reusing the
> >  	 * key to encrypt updated data. Simply put: one key, one encryption.
> >  	 */
> > -	u8 zero_nonce[crypto_aead_ivsize(big_key_aead)];
> > +	u8 zero_nonce[GCM_AES_IV_SIZE];
> >  
> >  	aead_req = aead_request_alloc(big_key_aead, GFP_KERNEL);
> >  	if (!aead_req)
> > @@ -425,6 +426,12 @@ static int __init big_key_init(void)
> >  		pr_err("Can't alloc crypto: %d\n", ret);
> >  		return ret;
> >  	}
> > +
> > +	if (unlikely(crypto_aead_ivsize(big_key_aead) != GCM_AES_IV_SIZE)) {
> > +		WARN(1, "big key algorithm changed?");
> > +		return -EINVAL;
> > +	}
> > +
> 
> 'big_key_aead' needs to be freed on error.
> 
> 	err = -EINVAL;
> 	goto free_aead;

oof, yes, thanks.

> Also how about defining the IV size next to the algorithm name?
> Then all the algorithm details would be on adjacent lines:
> 
> static const char big_key_alg_name[] = "gcm(aes)";
> #define BIG_KEY_IV_SIZE         GCM_AES_IV_SIZE

Sounds good, I'll fix both of these for v3.

Cheers,

Tycho

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