[PATCH 77/82] virtio: 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: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
Cc: Jason Wang <jasowang@xxxxxxxxxx>
Cc: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
Cc: virtualization@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 drivers/virtio/virtio_pci_modern_dev.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/virtio/virtio_pci_modern_dev.c b/drivers/virtio/virtio_pci_modern_dev.c
index 0d3dbfaf4b23..710d3bd45b4f 100644
--- a/drivers/virtio/virtio_pci_modern_dev.c
+++ b/drivers/virtio/virtio_pci_modern_dev.c
@@ -59,7 +59,7 @@ vp_modern_map_capability(struct virtio_pci_modern_device *mdev, int off,
 
 	length -= start;
 
-	if (start + offset < offset) {
+	if (add_would_overflow(offset, start)) {
 		dev_err(&dev->dev,
 			"virtio_pci: map wrap-around %u+%u\n",
 			start, offset);
@@ -81,7 +81,7 @@ vp_modern_map_capability(struct virtio_pci_modern_device *mdev, int off,
 	if (len)
 		*len = length;
 
-	if (minlen + offset < minlen ||
+	if (add_would_overflow(minlen, offset) ||
 	    minlen + offset > pci_resource_len(dev, bar)) {
 		dev_err(&dev->dev,
 			"virtio_pci: map virtio %zu@%u "
-- 
2.34.1