[PATCH 28/82] niu: Refactor intentional wrap-around calculation

[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 unsigned wrap-around addition test to use
check_add_overflow(), retaining the result for later usage (which removes
the redundant open-coded addition). This paves the way to enabling the
unsigned wrap-around sanitizer[2] 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: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
Cc: Simon Horman <horms@xxxxxxxxxx>
Cc: Du Cheng <ducheng2@xxxxxxxxx>
Cc: netdev@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 drivers/net/ethernet/sun/niu.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/sun/niu.c b/drivers/net/ethernet/sun/niu.c
index 21431f43e4c2..a4de07c6e618 100644
--- a/drivers/net/ethernet/sun/niu.c
+++ b/drivers/net/ethernet/sun/niu.c
@@ -6877,15 +6877,16 @@ static int niu_get_eeprom(struct net_device *dev,
 {
 	struct niu *np = netdev_priv(dev);
 	u32 offset, len, val;
+	u32 sum;
 
 	offset = eeprom->offset;
 	len = eeprom->len;
 
-	if (offset + len < offset)
+	if (check_add_overflow(offset, len, &sum))
 		return -EINVAL;
 	if (offset >= np->eeprom_len)
 		return -EINVAL;
-	if (offset + len > np->eeprom_len)
+	if (sum > np->eeprom_len)
 		len = eeprom->len = np->eeprom_len - offset;
 
 	if (offset & 3) {
-- 
2.34.1