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@xxxxxxxxxxxxx> Cc: Jozsef Kadlecsik <kadlec@xxxxxxxxxxxxx> Cc: Florian Westphal <fw@xxxxxxxxx> Cc: "David S. Miller" <davem@xxxxxxxxxxxxx> Cc: Eric Dumazet <edumazet@xxxxxxxxxx> Cc: Jakub Kicinski <kuba@xxxxxxxxxx> Cc: Paolo Abeni <pabeni@xxxxxxxxxx> Cc: netfilter-devel@xxxxxxxxxxxxxxx Cc: coreteam@xxxxxxxxxxxxx Cc: netdev@xxxxxxxxxxxxxxx Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> --- 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
