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

[Eric Auger]

On 1/23/24 11:49, Marc Zyngier wrote:
> 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.
I do agree.
>
>> +
>> +	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>
assuming above suggested changes,

Reviewed-by: Eric Auger <eric.auger@xxxxxxxxxx>

Eric
>
> 	M.
>