[PATCH 35/82] ACPI: custom_method: 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: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>
Cc: Len Brown <lenb@xxxxxxxxxx>
Cc: linux-acpi@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 drivers/acpi/custom_method.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/acpi/custom_method.c b/drivers/acpi/custom_method.c
index d39a9b474727..0789317f4a1a 100644
--- a/drivers/acpi/custom_method.c
+++ b/drivers/acpi/custom_method.c
@@ -54,7 +54,7 @@ static ssize_t cm_write(struct file *file, const char __user *user_buf,
 
 	if ((*ppos > max_size) ||
 	    (*ppos + count > max_size) ||
-	    (*ppos + count < count) ||
+	    (add_would_overflow(count, *ppos)) ||
 	    (count > uncopied_bytes)) {
 		kfree(buf);
 		buf = NULL;
-- 
2.34.1