[PATCH 24/82] KVM: arm64: vgic: 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: Marc Zyngier <maz at kernel.org>
Cc: Oliver Upton <oliver.upton at linux.dev>
Cc: James Morse <james.morse at arm.com>
Cc: Suzuki K Poulose <suzuki.poulose at arm.com>
Cc: Zenghui Yu <yuzenghui at huawei.com>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Will Deacon <will at kernel.org>
Cc: Reiji Watanabe <reijiw at google.com>
Cc: Eric Auger <eric.auger at redhat.com>
Cc: Ricardo Koller <ricarkol at google.com>
Cc: Raghavendra Rao Ananta <rananta at google.com>
Cc: Quentin Perret <qperret at google.com>
Cc: Jean-Philippe Brucker <jean-philippe at linaro.org>
Cc: linux-arm-kernel at lists.infradead.org
Cc: kvmarm at lists.linux.dev
Signed-off-by: Kees Cook <keescook at chromium.org>
---
 arch/arm64/kvm/vgic/vgic-kvm-device.c |  6 ++++--
 arch/arm64/kvm/vgic/vgic-v2.c         | 10 ++++++----
 2 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kvm/vgic/vgic-kvm-device.c b/arch/arm64/kvm/vgic/vgic-kvm-device.c
index f48b8dab8b3d..0eec5344d203 100644
--- a/arch/arm64/kvm/vgic/vgic-kvm-device.c
+++ b/arch/arm64/kvm/vgic/vgic-kvm-device.c
@@ -18,17 +18,19 @@ int vgic_check_iorange(struct kvm *kvm, phys_addr_t ioaddr,
 		       phys_addr_t addr, phys_addr_t alignment,
 		       phys_addr_t size)
 {
+	phys_addr_t sum;
+
 	if (!IS_VGIC_ADDR_UNDEF(ioaddr))
 		return -EEXIST;
 
 	if (!IS_ALIGNED(addr, alignment) || !IS_ALIGNED(size, alignment))
 		return -EINVAL;
 
-	if (addr + size < addr)
+	if (check_add_overflow(addr, size, &sum))
 		return -EINVAL;
 
 	if (addr & ~kvm_phys_mask(&kvm->arch.mmu) ||
-	    (addr + size) > kvm_phys_size(&kvm->arch.mmu))
+	    sum > kvm_phys_size(&kvm->arch.mmu))
 		return -E2BIG;
 
 	return 0;
diff --git a/arch/arm64/kvm/vgic/vgic-v2.c b/arch/arm64/kvm/vgic/vgic-v2.c
index 7e9cdb78f7ce..c8d1e965d3b7 100644
--- a/arch/arm64/kvm/vgic/vgic-v2.c
+++ b/arch/arm64/kvm/vgic/vgic-v2.c
@@ -273,14 +273,16 @@ void vgic_v2_enable(struct kvm_vcpu *vcpu)
 /* check for overlapping regions and for regions crossing the end of memory */
 static bool vgic_v2_check_base(gpa_t dist_base, gpa_t cpu_base)
 {
-	if (dist_base + KVM_VGIC_V2_DIST_SIZE < dist_base)
+	gpa_t dist_sum, cpu_sum;
+
+	if (check_add_overflow(dist_base, KVM_VGIC_V2_DIST_SIZE, &dist_sum))
 		return false;
-	if (cpu_base + KVM_VGIC_V2_CPU_SIZE < cpu_base)
+	if (check_add_overflow(cpu_base, KVM_VGIC_V2_CPU_SIZE, &cpu_sum))
 		return false;
 
-	if (dist_base + KVM_VGIC_V2_DIST_SIZE <= cpu_base)
+	if (dist_sum <= cpu_base)
 		return true;
-	if (cpu_base + KVM_VGIC_V2_CPU_SIZE <= dist_base)
+	if (cpu_sum <= dist_base)
 		return true;
 
 	return false;
-- 
2.34.1