[PATCH 32/82] vringh: Refactor intentional wrap-around calculation

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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: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
Cc: Jason Wang <jasowang@xxxxxxxxxx>
Cc: kvm@xxxxxxxxxxxxxxx
Cc: virtualization@xxxxxxxxxxxxxxx
Cc: netdev@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
 drivers/vhost/vringh.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
index 7b8fd977f71c..07442f0a52bd 100644
--- a/drivers/vhost/vringh.c
+++ b/drivers/vhost/vringh.c
@@ -145,6 +145,8 @@ static inline bool range_check(struct vringh *vrh, u64 addr, size_t *len,
 			       bool (*getrange)(struct vringh *,
 						u64, struct vringh_range *))
+	u64 sum;
 	if (addr < range->start || addr > range->end_incl) {
 		if (!getrange(vrh, addr, range))
 			return false;
@@ -152,20 +154,20 @@ static inline bool range_check(struct vringh *vrh, u64 addr, size_t *len,
 	BUG_ON(addr < range->start || addr > range->end_incl);
 	/* To end of memory? */
-	if (unlikely(addr + *len == 0)) {
+	if (unlikely(U64_MAX - addr == *len)) {
 		if (range->end_incl == -1ULL)
 			return true;
 		goto truncate;
 	/* Otherwise, don't wrap. */
-	if (addr + *len < addr) {
+	if (check_add_overflow(addr, *len, &sum)) {
 		vringh_bad("Wrapping descriptor %zu@0x%llx",
 			   *len, (unsigned long long)addr);
 		return false;
-	if (unlikely(addr + *len - 1 > range->end_incl))
+	if (unlikely(sum - 1 > range->end_incl))
 		goto truncate;
 	return true;

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