* [PATCH 64/82] netfilter: Refactor intentional wrap-around test
[not found] <20240122235208.work.748-kees@kernel.org>
@ 2024-01-23 0:27 ` Kees Cook
2024-01-23 18:03 ` Florian Westphal
0 siblings, 1 reply; 2+ messages in thread
From: Kees Cook @ 2024-01-23 0:27 UTC (permalink / raw)
To: linux-hardening
Cc: Kees Cook, Pablo Neira Ayuso, Jozsef Kadlecsik, Florian Westphal,
David S. Miller, Eric Dumazet, Jakub Kicinski, Paolo Abeni,
netfilter-devel, coreteam, netdev, Gustavo A. R. Silva,
Bill Wendling, Justin Stitt, linux-kernel
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
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