[PATCH 16/82] drm/nouveau/mmu: 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: Karol Herbst <kherbst@xxxxxxxxxx>
Cc: Lyude Paul <lyude@xxxxxxxxxx>
Cc: Danilo Krummrich <dakr@xxxxxxxxxx>
Cc: David Airlie <airlied@xxxxxxxxx>
Cc: Daniel Vetter <daniel@xxxxxxxx>
Cc: Ben Skeggs <bskeggs@xxxxxxxxxx>
Cc: Dave Airlie <airlied@xxxxxxxxxx>
Cc: Julia Lawall <Julia.Lawall@xxxxxxxx>
Cc: Jiang Jian <jiangjian@xxxxxxxxxx>
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: nouveau@xxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 drivers/gpu/drm/nouveau/nvkm/subdev/mmu/vmm.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/nouveau/nvkm/subdev/mmu/vmm.c b/drivers/gpu/drm/nouveau/nvkm/subdev/mmu/vmm.c
index 9c97800fe037..6ca1a82ccbc1 100644
--- a/drivers/gpu/drm/nouveau/nvkm/subdev/mmu/vmm.c
+++ b/drivers/gpu/drm/nouveau/nvkm/subdev/mmu/vmm.c
@@ -1149,13 +1149,15 @@ nvkm_vmm_ctor(const struct nvkm_vmm_func *func, struct nvkm_mmu *mmu,
 	vmm->root = RB_ROOT;
 
 	if (managed) {
+		u64 sum;
+
 		/* Address-space will be managed by the client for the most
 		 * part, except for a specified area where NVKM allocations
 		 * are allowed to be placed.
 		 */
 		vmm->start = 0;
 		vmm->limit = 1ULL << bits;
-		if (addr + size < addr || addr + size > vmm->limit)
+		if (check_add_overflow(addr, size, &sum) || sum > vmm->limit)
 			return -EINVAL;
 
 		/* Client-managed area before the NVKM-managed area. */
@@ -1174,7 +1176,7 @@ nvkm_vmm_ctor(const struct nvkm_vmm_func *func, struct nvkm_mmu *mmu,
 		}
 
 		/* Client-managed area after the NVKM-managed area. */
-		addr = addr + size;
+		addr = sum;
 		size = vmm->limit - addr;
 		if (size && (ret = nvkm_vmm_ctor_managed(vmm, addr, size)))
 			return ret;
-- 
2.34.1