netdev - Re: [PATCH net 2/3] tls: clear key material from kernel memory when do_tls_setsockopt

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Message-ID: <20180906074953.GB14270@bistromath.localdomain>
Date:   Thu, 6 Sep 2018 09:49:53 +0200
From:   Sabrina Dubroca <sd@...asysnail.net>
To:     Boris Pismenny <borisp@...lanox.com>
Cc:     netdev@...r.kernel.org, Ilya Lesokhin <ilyal@...lanox.com>,
        Aviad Yehezkel <aviadye@...lanox.com>,
        Dave Watson <davejwatson@...com>
Subject: Re: [PATCH net 2/3] tls: clear key material from kernel memory when
 do_tls_setsockopt_conf fails

2018-09-05, 16:53:54 +0300, Boris Pismenny wrote:
> Hi Sabrina,
> 
> On 9/5/2018 4:21 PM, Sabrina Dubroca wrote:
> > Fixes: 3c4d7559159b ("tls: kernel TLS support")
> > Signed-off-by: Sabrina Dubroca <sd@...asysnail.net>
> > ---
> >   net/tls/tls_main.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/net/tls/tls_main.c b/net/tls/tls_main.c
> > index 180b6640e531..0d432d025471 100644
> > --- a/net/tls/tls_main.c
> > +++ b/net/tls/tls_main.c
> > @@ -499,7 +499,7 @@ static int do_tls_setsockopt_conf(struct sock *sk, char __user *optval,
> >   	goto out;
> >   err_crypto_info:
> > -	memset(crypto_info, 0, sizeof(*crypto_info));
> > +	memzero_explicit(crypto_info, sizeof(struct tls12_crypto_info_aes_gcm_128));
> 
> Besides the key, there are other (not secret) information in
> tls12_crypto_info_aes_gcm_128. I'd prefer you do not delete it to enable
> users to obtain it (using getsockopt) in case we decide to implement a
> fallback to userspace in the future. Such a fallback must obtain the
> kernel's iv, and record sequence number.

The operation failed. There's no reason for this stale data to remain
in the kernel. And you couldn't recover it with getsockopt, because
you'll hit !TLS_CRYPTO_INFO_READY, since we're already resetting
tls_crypto_info. Resetting tls_crypto_info on failure is necessary,
otherwise your socket will be in a broken state, with TLS not setup
but unable to perform a new attempt.

Userspace already has all this information anyway, since it passed it
to the kernel, so why would it need to recover it from the kernel?

-- 
Sabrina