Re: [Xen-devel] [PATCH] arm/vm_event: get/set registers

[Tamas K Lengyel <tamas.lengyel@xxxxxxxxxxxx>]

On Thu, Jul 28, 2016 at 2:26 PM, Andrew Cooper
<andrew.cooper3@xxxxxxxxxx> wrote:
> On 28/07/2016 21:05, Tamas K Lengyel wrote:
>> Add support for getting/setting registers through vm_event on ARM.
>> The set of registers can be expanded in the future to include other registers
>> as well if necessary but for now it is limited to TTB/CR/R0/R1, PC and CPSR.
>>
>> Signed-off-by: Tamas K Lengyel <tamas.lengyel@xxxxxxxxxxxx>
>> ---
>> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>> Cc: Julien Grall <julien.grall@xxxxxxx>
>> Cc: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
>> Cc: Jan Beulich <jbeulich@xxxxxxxx>
>> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>
> For the x86 and common bits, Reviewed-by: Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx>
>
> However,
>
>> +#include <xen/sched.h>
>> +#include <asm/vm_event.h>
>> +
>> +void vm_event_fill_regs(vm_event_request_t *req)
>> +{
>> +    const struct cpu_user_regs *regs = guest_cpu_user_regs();
>> +
>> +    req->data.regs.arm.cpsr = regs->cpsr;
>> +    req->data.regs.arm.pc = regs->pc;
>> +    req->data.regs.arm.ttbcr = READ_SYSREG(TCR_EL1);
>> +    req->data.regs.arm.ttbr0 = READ_SYSREG64(TTBR0_EL1);
>> +    req->data.regs.arm.ttbr1 = READ_SYSREG64(TTBR1_EL1);
>> +}
>> +
>> +void vm_event_set_registers(struct vcpu *v, vm_event_response_t *rsp)
>> +{
>> +    struct cpu_user_regs *regs = guest_cpu_user_regs();
>> +
>> +    regs->cpsr = rsp->data.regs.arm.cpsr;
>> +    regs->pc = rsp->data.regs.arm.pc;
>> +    v->arch.ttbcr = rsp->data.regs.arm.ttbcr;
>> +    v->arch.ttbr0 = rsp->data.regs.arm.ttbr0;
>> +    v->arch.ttbr1 = rsp->data.regs.arm.ttbr1;
>
> Not knowing anything about ARM, but this looks like it is missing some
> sanity/plausibility checks (to protect Xen against accidental clobbering
> from the vm_event listener), and some WRITE_SYSREG() to reload the new
> values (unless this is done unconditionally later, at which point you
> should at least leave a comment here saying so).
>

This function only ever gets called if the vm_event response
specifically has the VM_EVENT_FLAG_SET_REGISTERS set, so accidental
clobbering is not possible. Also, using WRITE_SYSREG() is not safe at
this point because current != v. However, I have another issue here
with regs which should actually be:

    struct cpu_user_regs *regs = &v->arch.cpu_info->guest_cpu_user_regs;

I'll fix that shortly.

Thanks,
Tamas

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