linux-kernel - [PATCH 80/82] xen-netback: Refactor intentional wrap-around test

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Message-Id: <20240123002814.1396804-80-keescook@chromium.org>
Date: Mon, 22 Jan 2024 16:27:55 -0800
From: Kees Cook <keescook@...omium.org>
To: linux-hardening@...r.kernel.org
Cc: Kees Cook <keescook@...omium.org>,
	Wei Liu <wei.liu@...nel.org>,
	Paul Durrant <paul@....org>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	xen-devel@...ts.xenproject.org,
	netdev@...r.kernel.org,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 80/82] xen-netback: Refactor intentional wrap-around test

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: Wei Liu <wei.liu@...nel.org>
Cc: Paul Durrant <paul@....org>
Cc: "David S. Miller" <davem@...emloft.net>
Cc: Eric Dumazet <edumazet@...gle.com>
Cc: Jakub Kicinski <kuba@...nel.org>
Cc: Paolo Abeni <pabeni@...hat.com>
Cc: xen-devel@...ts.xenproject.org
Cc: netdev@...r.kernel.org
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 drivers/net/xen-netback/hash.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/xen-netback/hash.c b/drivers/net/xen-netback/hash.c
index ff96f22648ef..69b03b4feba9 100644
--- a/drivers/net/xen-netback/hash.c
+++ b/drivers/net/xen-netback/hash.c
@@ -345,7 +345,7 @@ u32 xenvif_set_hash_mapping(struct xenvif *vif, u32 gref, u32 len,
 		.flags = GNTCOPY_source_gref
 	}};
 
-	if ((off + len < off) || (off + len > vif->hash.size) ||
+	if ((add_would_overflow(off, len)) || (off + len > vif->hash.size) ||
 	    len > XEN_PAGE_SIZE / sizeof(*mapping))
 		return XEN_NETIF_CTRL_STATUS_INVALID_PARAMETER;
 
-- 
2.34.1