[PATCH 64/82] netfilter: Refactor intentional wrap-around test

[PATCH 64/82] netfilter: Refactor intentional wrap-around test

[Kees Cook]

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

Re: [PATCH 64/82] netfilter: Refactor intentional wrap-around test

[Florian Westphal]

Kees Cook <keescook@chromium.org> wrote:
> 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

Acked-by: Florian Westphal <fw@strlen.de>