Received-SPF: pass (google.com: domain of linux-kernel+bounces-34489-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45d1:ec00::1 as permitted sender) client-ip=2604:1380:45d1:ec00::1;
From: Kees Cook <keescook@chromium.org>
To: linux-hardening@vger.kernel.org
Cc: Kees Cook <keescook@chromium.org>,
	Pablo Neira Ayuso <pablo@netfilter.org>,
	Jozsef Kadlecsik <kadlec@netfilter.org>,
	Florian Westphal <fw@strlen.de>,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>,
	Paolo Abeni <pabeni@redhat.com>,
	netfilter-devel@vger.kernel.org,
	coreteam@netfilter.org,
	netdev@vger.kernel.org,
	"Gustavo A. R. Silva" <gustavoars@kernel.org>,
	Bill Wendling <morbo@google.com>,
	Justin Stitt <justinstitt@google.com>,
	linux-kernel@vger.kernel.org
Subject: [PATCH 64/82] netfilter: Refactor intentional wrap-around test
Date: Mon, 22 Jan 2024 16:27:39 -0800
Message-Id: <20240123002814.1396804-64-keescook@chromium.org>
In-Reply-To: <20240122235208.work.748-kees@kernel.org>
References: <20240122235208.work.748-kees@kernel.org>
Precedence: bulk
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit

In an effort to separate intentional arithmetic wrap-around from
unexpected wrap-around, we need to refactor places that depend on this
kind of math. One of the most common code patterns of this is:

	VAR + value < VAR

Notably, this is considered "undefined behavior" for signed and pointer
types, which the kernel works around by using the -fno-strict-overflow
option in the build[1] (which used to just be -fwrapv). Regardless, we
want to get the kernel source to the position where we can meaningfully
instrument arithmetic wrap-around conditions and catch them when they
are unexpected, regardless of whether they are signed[2], unsigned[3],
or pointer[4] types.

Refactor open-coded wrap-around addition test to use add_would_overflow().
This paves the way to enabling the wrap-around sanitizers in the future.

Link: https://git.kernel.org/linus/68df3755e383e6fecf2354a67b08f92f18536594 [1]
Link: https://github.com/KSPP/linux/issues/26 [2]
Link: https://github.com/KSPP/linux/issues/27 [3]
Link: https://github.com/KSPP/linux/issues/344 [4]
Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: Jozsef Kadlecsik <kadlec@netfilter.org>
Cc: Florian Westphal <fw@strlen.de>
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: netfilter-devel@vger.kernel.org
Cc: coreteam@netfilter.org
Cc: netdev@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 net/netfilter/xt_u32.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/netfilter/xt_u32.c b/net/netfilter/xt_u32.c
index 117d4615d668..8623fe2d97e9 100644
--- a/net/netfilter/xt_u32.c
+++ b/net/netfilter/xt_u32.c
@@ -58,11 +58,11 @@ static bool u32_match_it(const struct xt_u32 *data,
 				val >>= number;
 				break;
 			case XT_U32_AT:
-				if (at + val < at)
+				if (add_would_overflow(at, val))
 					return false;
 				at += val;
 				pos = number;
-				if (at + 4 < at || skb->len < at + 4 ||
+				if (add_would_overflow(at, 4) || skb->len < at + 4 ||
 				    pos > skb->len - at - 4)
 					return false;
 
-- 
2.34.1