Re: [PATCH net 3/3] tls: zero the crypto information from tls_context before freeing

[Sabrina Dubroca <sd@queasysnail.net>]

2018-09-05, 13:48:48 +0000, Vakul Garg wrote:
> 
> 
> > -----Original Message-----
> > From: netdev-owner@vger.kernel.org <netdev-owner@vger.kernel.org> On
> > Behalf Of Sabrina Dubroca
> > Sent: Wednesday, September 5, 2018 6:52 PM
> > To: netdev@vger.kernel.org
> > Cc: Sabrina Dubroca <sd@queasysnail.net>; Boris Pismenny
> > <borisp@mellanox.com>; Ilya Lesokhin <ilyal@mellanox.com>; Aviad
> > Yehezkel <aviadye@mellanox.com>; Dave Watson <davejwatson@fb.com>
> > Subject: [PATCH net 3/3] tls: zero the crypto information from tls_context
> > before freeing
> > 
> > This contains key material in crypto_send_aes_gcm_128 and
> > crypto_recv_aes_gcm_128.
> > 
> > Fixes: 3c4d7559159b ("tls: kernel TLS support")
> > Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
> > ---
> >  include/net/tls.h  |  1 +
> >  net/tls/tls_main.c | 14 ++++++++++++--
> >  2 files changed, 13 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/net/tls.h b/include/net/tls.h index
> > d5c683e8bb22..2010d23112f9 100644
> > --- a/include/net/tls.h
> > +++ b/include/net/tls.h
> > @@ -180,6 +180,7 @@ struct tls_context {
> >  		struct tls_crypto_info crypto_recv;
> >  		struct tls12_crypto_info_aes_gcm_128
> > crypto_recv_aes_gcm_128;
> >  	};
> > +	char tls_crypto_ctx_end[0];
> 
> This makes the key zeroization dependent upon the position of unions
> in structure.

Yes. I could add char tls_crypto_ctx_start[0].

> Can you zeroise the crypto_send, crypto_recv separately using two
> memzero_explicit calls?

It's not crypto_send, it's crypto_send_aes_gcm_128. I don't know how
likely it is that another crypto algorithm will ever be added, but
then you'd have to switch to zeroing that thing.

I had a different version of the patch that gave a name to those
unions, but then I need to change all the references everywhere and
the patch becomes a bit ugly, especially for net.

struct tls_context {
	union {
		struct tls_crypto_info info;
		struct tls12_crypto_info_aes_gcm_128 aes_gcm_128;
	} crypto_send;
	union {
		struct tls_crypto_info info;
		struct tls12_crypto_info_aes_gcm_128 aes_gcm_128;
	} crypto_recv;

	[...]
}


Or even:

union tls_crypto_context {
	struct tls_crypto_info info;
	struct tls12_crypto_info_aes_gcm_128 aes_gcm_128;
};

struct tls_context {
	union tls_crypto_context crypto_send;
	union tls_crypto_context crypto_recv;

	[...]
}


-- 
Sabrina