Re: [PATCH] QEMU-KVM: MCE: Relay UCR MCE to guest

[Avi Kivity <avi@redhat.com>]

On 09/07/2009 11:48 PM, Anthony Liguori wrote:
>
>>  #ifdef KVM_CAP_IRQCHIP
>>
>>  int kvm_set_irq_level(kvm_context_t kvm, int irq, int level, int 
>> *status)
>> @@ -1515,6 +1546,38 @@ static void sig_ipi_handler(int n)
>>  {
>>  }
>>
>> +static void sigbus_handler(int n, struct signalfd_siginfo *siginfo, 
>> void *ctx)
>> +{
>> +    if (siginfo->ssi_code == BUS_MCEERR_AO) {
>> +        uint64_t status;
>> +        unsigned long paddr;
>> +        CPUState *cenv;
>> +
>> +        /* Hope we are lucky for AO MCE */
>
> Even if the error was limited to guest memory, it could have been 
> generated by either the kernel or userspace reading guest memory, no?
>
> Does this potentially open a security hole for us?  Consider the 
> following:
>
> 1) We happen to read guest memory and that causes an MCE.  For 
> instance, say we're in virtio.c and we read the virtio ring.
> 2) That should trigger the kernel to generate a sigbus.
> 3) We catch sigbus, and queue an MCE for delivery.
> 4) After sigbus handler completes, we're back in virtio.c, what was 
> the value of the memory operation we just completed?
>
> If the instruction gets skipped, we may be leaking host memory because 
> the access never happened.
>

I think it's a lot safer to only report guest mode accesses to the 
guest, and let user mode accesses terminate qemu.  The guest wouldn't 
expect 100% recovery; for example if an uncorrectable error hit a vital 
kernel data structure.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.