Re: [PATCH V3 2/2] xen: Introduce VM_EVENT_FLAG_SET_REGISTERS

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 28/09/15 16:25, Jan Beulich wrote:
>>>> On 28.09.15 at 12:16, <rcojocaru@bitdefender.com> wrote:
>> +void vm_event_set_registers(struct vcpu *v, vm_event_response_t *rsp)
>> +{
>> +    v->arch.user_regs.eax = rsp->data.regs.x86.rax;
>> +    v->arch.user_regs.ebx = rsp->data.regs.x86.rbx;
>> +    v->arch.user_regs.ecx = rsp->data.regs.x86.rcx;
>> +    v->arch.user_regs.edx = rsp->data.regs.x86.rdx;
>> +    v->arch.user_regs.esp = rsp->data.regs.x86.rsp;
>> +    v->arch.user_regs.ebp = rsp->data.regs.x86.rbp;
>> +    v->arch.user_regs.esi = rsp->data.regs.x86.rsi;
>> +    v->arch.user_regs.edi = rsp->data.regs.x86.rdi;
>> +
>> +    v->arch.user_regs.r8 = rsp->data.regs.x86.r8;
>> +    v->arch.user_regs.r9 = rsp->data.regs.x86.r9;
>> +    v->arch.user_regs.r10 = rsp->data.regs.x86.r10;
>> +    v->arch.user_regs.r11 = rsp->data.regs.x86.r11;
>> +    v->arch.user_regs.r12 = rsp->data.regs.x86.r12;
>> +    v->arch.user_regs.r13 = rsp->data.regs.x86.r13;
>> +    v->arch.user_regs.r14 = rsp->data.regs.x86.r14;
>> +    v->arch.user_regs.r15 = rsp->data.regs.x86.r15;
>> +
>> +    v->arch.user_regs.eflags = rsp->data.regs.x86.rflags;
> Shouldn't you sanitize the value? I can't immediately see anything
> putting Xen at risk (but it also doesn't seem impossible that I'm
> overlooking something), but surely putting insane values here
> can lead to hard to debug guest crashes.

I had the same thought (e.g. XSA-111), but all modifications like this
are already possible with a cunningly-crafted sethvmcontext so we are at
no more risk than before.

Furthermore, I can't think of any plausible validation which could be
done.  It is entirely possible that this interface could be used to
bounce execution into a hidden introspection agent.

~Andrew