[PATCH 72/82] scsi: sd_zbc: Refactor intentional wrap-around test

[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 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: "James E.J. Bottomley" <jejb@xxxxxxxxxxxxx>
Cc: "Martin K. Petersen" <martin.petersen@xxxxxxxxxx>
Cc: linux-scsi@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 drivers/scsi/sd_zbc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/scsi/sd_zbc.c b/drivers/scsi/sd_zbc.c
index 26af5ab7d7c1..2c377e4cdb2b 100644
--- a/drivers/scsi/sd_zbc.c
+++ b/drivers/scsi/sd_zbc.c
@@ -295,7 +295,7 @@ int sd_zbc_report_zones(struct gendisk *disk, sector_t sector,
 			    (lba < start_lba ||
 			     lba >= start_lba + zone_length)) ||
 			    (zone_idx > 0 && start_lba != lba) ||
-			    start_lba + zone_length < start_lba) {
+			    add_would_overflow(start_lba, zone_length)) {
 				sd_printk(KERN_ERR, sdkp,
 					  "Zone %d at LBA %llu is invalid: %llu + %llu\n",
 					  zone_idx, lba, start_lba, zone_length);
-- 
2.34.1