Re: [PATCH] wifi: nl80211: Split memcpy() of struct nl80211_wowlan_tcp_data_token flexible array

From: Kees Cook <keescook@chromium.org>
To: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: Johannes Berg <johannes@sipsolutions.net>,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	linux-wireless@vger.kernel.org, netdev@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
Subject: Re: [PATCH] wifi: nl80211: Split memcpy() of struct nl80211_wowlan_tcp_data_token flexible array
Date: Mon, 26 Sep 2022 19:27:34 -0700	[thread overview]
Message-ID: <202209261924.BE1E0F29FA@keescook> (raw)
In-Reply-To: <YzJS+wRX0nu3qE4F@work>

On Mon, Sep 26, 2022 at 08:33:47PM -0500, Gustavo A. R. Silva wrote:
> On Mon, Sep 26, 2022 at 05:39:03PM -0700, Kees Cook wrote:
> > To work around a misbehavior of the compiler's ability to see into
> > composite flexible array structs (as detailed in the coming memcpy()
> > hardening series[1]), split the memcpy() of the header and the payload
> > so no false positive run-time overflow warning will be generated.
> > 
> > [1] https://lore.kernel.org/linux-hardening/20220901065914.1417829-2-keescook@chromium.org/
> > 
> > Cc: Johannes Berg <johannes@sipsolutions.net>
> > Cc: "David S. Miller" <davem@davemloft.net>
> > Cc: Eric Dumazet <edumazet@google.com>
> > Cc: Jakub Kicinski <kuba@kernel.org>
> > Cc: Paolo Abeni <pabeni@redhat.com>
> > Cc: linux-wireless@vger.kernel.org
> > Cc: netdev@vger.kernel.org
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> >  net/wireless/nl80211.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/net/wireless/nl80211.c b/net/wireless/nl80211.c
> > index 2705e3ee8fc4..461897933e92 100644
> > --- a/net/wireless/nl80211.c
> > +++ b/net/wireless/nl80211.c
> > @@ -13171,7 +13171,8 @@ static int nl80211_parse_wowlan_tcp(struct cfg80211_registered_device *rdev,
> >  	       wake_mask_size);
> >  	if (tok) {
> >  		cfg->tokens_size = tokens_size;
> > -		memcpy(&cfg->payload_tok, tok, sizeof(*tok) + tokens_size);
> > +		cfg->payload_tok = *tok;
> > +		memcpy(cfg->payload_tok.token_stream, tok->token_stream, + tokens_size);
> 
> There is a spurious '+' here............................................^^^

HUnh. Well. I learned a new C-ism today. A leading "+" is just accepted.
I build-tested this, and it was happy. I just double-checked, and sure
enough, it is actually parsing that line. I will send a v2. Thanks for
catching that! ... That's such a weird typo we don't even have a
checkpatch rule for it. :P

-- 
Kees Cook