[PATCH 17/82] drm/vc4: Refactor intentional wrap-around calculation

[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 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
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: Maxime Ripard <mripard@xxxxxxxxxx>
Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
Cc: Thomas Zimmermann <tzimmermann@xxxxxxx>
Cc: David Airlie <airlied@xxxxxxxxx>
Cc: Daniel Vetter <daniel@xxxxxxxx>
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 drivers/gpu/drm/vc4/vc4_validate.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/vc4/vc4_validate.c b/drivers/gpu/drm/vc4/vc4_validate.c
index 7dff3ca5af6b..9affba9c58b3 100644
--- a/drivers/gpu/drm/vc4/vc4_validate.c
+++ b/drivers/gpu/drm/vc4/vc4_validate.c
@@ -305,6 +305,7 @@ validate_gl_array_primitive(VALIDATE_ARGS)
 	uint32_t length = *(uint32_t *)(untrusted + 1);
 	uint32_t base_index = *(uint32_t *)(untrusted + 5);
 	uint32_t max_index;
+	uint32_t sum;
 	struct vc4_shader_state *shader_state;
 
 	/* Check overflow condition */
@@ -314,11 +315,11 @@ validate_gl_array_primitive(VALIDATE_ARGS)
 	}
 	shader_state = &exec->shader_state[exec->shader_state_count - 1];
 
-	if (length + base_index < length) {
+	if (check_add_overflow(length, base_index, &sum)) {
 		DRM_DEBUG("primitive vertex count overflow\n");
 		return -EINVAL;
 	}
-	max_index = length + base_index - 1;
+	max_index = sum - 1;
 
 	if (max_index > shader_state->max_index)
 		shader_state->max_index = max_index;
-- 
2.34.1