[PATCH 36/82] agp: 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: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: David Airlie <airlied@xxxxxxxxxx>
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 drivers/char/agp/generic.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/char/agp/generic.c b/drivers/char/agp/generic.c
index 3ffbb1c80c5c..fc2d07654154 100644
--- a/drivers/char/agp/generic.c
+++ b/drivers/char/agp/generic.c
@@ -228,7 +228,7 @@ struct agp_memory *agp_allocate_memory(struct agp_bridge_data *bridge,
 
 	cur_memory = atomic_read(&bridge->current_memory_agp);
 	if ((cur_memory + page_count > bridge->max_memory_agp) ||
-	    (cur_memory + page_count < page_count))
+	    (add_would_overflow(page_count, cur_memory)))
 		return NULL;
 
 	if (type >= AGP_USER_TYPES) {
-- 
2.34.1