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
