Re: [PATCH 24/82] KVM: arm64: vgic: Refactor intentional wrap-around calculation

[Marc Zyngier]

On Tue, 23 Jan 2024 00:26:59 +0000,
Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> 
> 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@xxxxxxxxxx>
> Cc: Oliver Upton <oliver.upton@xxxxxxxxx>
> Cc: James Morse <james.morse@xxxxxxx>
> Cc: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> Cc: Zenghui Yu <yuzenghui@xxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Reiji Watanabe <reijiw@xxxxxxxxxx>
> Cc: Eric Auger <eric.auger@xxxxxxxxxx>
> Cc: Ricardo Koller <ricarkol@xxxxxxxxxx>
> Cc: Raghavendra Rao Ananta <rananta@xxxxxxxxxx>
> Cc: Quentin Perret <qperret@xxxxxxxxxx>
> Cc: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: kvmarm@xxxxxxxxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  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))

nit: 'sum' doesn't mean much in this context. Something like 'end'
would be much more descriptive.

>  		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;

Same here: dist_end, cpu_end.

> +
> +	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;

With these nits addressed, and assuming you intend to merge the whole
series yourself:

Acked-by: Marc Zyngier <maz@xxxxxxxxxx>

	M.

-- 
Without deviation from the norm, progress is not possible.