[PATCH 21/82] drivers/fsi: 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
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: Jeremy Kerr <jk@xxxxxxxxxx>
Cc: Joel Stanley <joel@xxxxxxxxx>
Cc: Alistar Popple <alistair@xxxxxxxxxxxx>
Cc: Eddie James <eajames@xxxxxxxxxxxxx>
Cc: linux-fsi@xxxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 drivers/fsi/fsi-core.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/fsi/fsi-core.c b/drivers/fsi/fsi-core.c
index 097d5a780264..46b24d0aadc6 100644
--- a/drivers/fsi/fsi-core.c
+++ b/drivers/fsi/fsi-core.c
@@ -381,10 +381,12 @@ EXPORT_SYMBOL_GPL(fsi_slave_write);
 int fsi_slave_claim_range(struct fsi_slave *slave,
 			  uint32_t addr, uint32_t size)
 {
-	if (addr + size < addr)
+	uint32_t sum;
+
+	if (check_add_overflow(addr, size, &sum))
 		return -EINVAL;
 
-	if (addr + size > slave->size)
+	if (sum > slave->size)
 		return -EINVAL;
 
 	/* todo: check for overlapping claims */
-- 
2.34.1