Re: [kernel-hardening] [PATCH 1/2] kvm: whitelist struct kvm_vcpu_arch

[Kees Cook]

On Mon, Oct 30, 2017 at 4:28 PM, Eric Biggers <ebiggers3@xxxxxxxxx> wrote:
> On Thu, Oct 26, 2017 at 03:45:46PM +0200, Paolo Bonzini wrote:
>> On x86, ARM and s390, struct kvm_vcpu_arch has a usercopy region
>> taht is read and written by the KVM_GET/SET_CPUID2 ioctls (x86)
>> or KVM_GET/SET_ONE_REG (ARM/s390).  Without whitelisting the area,
>> KVM is completely broken on those architectures with usercopy hardening
>> enabled.
>>
>> For now, allow writing to the entire struct on all architectures.
>> The KVM tree will not refine this to an architecture-specific
>> subset of struct kvm_vcpu_arch.
>>
>> Cc: kernel-hardening@xxxxxxxxxxxxxxxxxx
>> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
>> Cc: Christian Borntraeger <borntraeger@xxxxxxxxxx>
>> Cc: Christoffer Dall <cdall@xxxxxxxxxx>
>> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
>> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>> ---
>>  virt/kvm/kvm_main.c | 8 ++++++--
>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>> index 4d81f6ded88e..b4809ccfdfa1 100644
>> --- a/virt/kvm/kvm_main.c
>> +++ b/virt/kvm/kvm_main.c
>> @@ -4005,8 +4005,12 @@ int kvm_init(void *opaque, unsigned vcpu_size, unsigned vcpu_align,
>>       /* A kmem cache lets us meet the alignment requirements of fx_save. */
>>       if (!vcpu_align)
>>               vcpu_align = __alignof__(struct kvm_vcpu);
>> -     kvm_vcpu_cache = kmem_cache_create("kvm_vcpu", vcpu_size, vcpu_align,
>> -                                        0, NULL);
>> +     kvm_vcpu_cache =
>> +             kmem_cache_create_usercopy("kvm_vcpu",
>> +                                        sizeof(struct kvm_vcpu), vcpu_align,
>> +                                        0, offsetof(struct kvm_vcpu, arch),
>> +                                        sizeof_field(struct kvm_vcpu, arch),
>> +                                        NULL);
>
> Doesn't it need to be 'vcpu_size' instead of 'sizeof(struct kvm_vcpu)'?

Oh, yikes, yes.

$ git grep '\bkvm_init('
arch/mips/kvm/mips.c:   ret = kvm_init(NULL, sizeof(struct kvm_vcpu),
0, THIS_MODULE);
arch/powerpc/kvm/book3s.c:      r = kvm_init(NULL, sizeof(struct
kvm_vcpu), 0, THIS_MODULE);
arch/powerpc/kvm/e500.c:        r = kvm_init(NULL, sizeof(struct
kvmppc_vcpu_e500), 0, THIS_MODULE);
arch/powerpc/kvm/e500mc.c:      r = kvm_init(NULL, sizeof(struct
kvmppc_vcpu_e500), 0, THIS_MODULE);
arch/s390/kvm/kvm-s390.c:       return kvm_init(NULL, sizeof(struct
kvm_vcpu), 0, THIS_MODULE);
arch/x86/kvm/svm.c:     return kvm_init(&svm_x86_ops, sizeof(struct vcpu_svm),
arch/x86/kvm/vmx.c:     int r = kvm_init(&vmx_x86_ops, sizeof(struct vcpu_vmx),
include/linux/kvm_host.h:int kvm_init(void *opaque, unsigned
vcpu_size, unsigned vcpu_align,
virt/kvm/arm/arm.c:     int rc = kvm_init(NULL, sizeof(struct
kvm_vcpu), 0, THIS_MODULE);

I'll fix this up.

-Kees

-- 
Kees Cook
Pixel Security