Re: [Qemu-devel] [RFC PATCH v1 10/22] sev: add SEV debug decrypt command

[Paolo Bonzini]

On 14/09/2016 04:28, Michael S. Tsirkin wrote:
> On Tue, Sep 13, 2016 at 10:48:27AM -0400, Brijesh Singh wrote:
>> The SEV DEBUG_DECRYPT command is used for decrypting a guest memory
>> for the debugging purposes. Note that debugging is permitting only
>> when guest policy allows it.
> 
> When wouldn't you want to allow it?
> I don't see value in a "break debugging" feature.

One man's "allow debugging" feature is another man's "break encryption"
feature. :)

Paolo

> 
>> For more information see [1], section 7.1
>>
>> [1] http://support.amd.com/TechDocs/55766_SEV-KM%20API_Spec.pdf
> 
> Please add comments documenting APIs. Spec links to figure out
> implementation is one thing, but you really can't require people
> to read specs just to figure out how to use an API.
> 
>> The following KVM RFC patches defines and implements this command
>>
>> http://marc.info/?l=kvm&m=147190852423972&w=2
>> http://marc.info/?l=kvm&m=147191068524579&w=2
>>
>> Signed-off-by: Brijesh Singh <address@hidden>
>> ---
>>  include/sysemu/sev.h |   10 ++++++++++
>>  sev.c                |   23 +++++++++++++++++++++++
>>  2 files changed, 33 insertions(+)
>>
>> diff --git a/include/sysemu/sev.h b/include/sysemu/sev.h
>> index ab03c5d..5872c3e 100644
>> --- a/include/sysemu/sev.h
>> +++ b/include/sysemu/sev.h
>> @@ -55,4 +55,14 @@ int kvm_sev_guest_finish(void);
>>   */
>>  int kvm_sev_guest_measurement(uint8_t *measurement);
>>  
>> +/**
>> + * kvm_sev_dbg_decrypt -  decrypt the guest memory for debugging purposes
>> + * @src - guest memory address
>> + * @dest - host memory address where the decrypted data should be copied
>> + * @length - length of memory region
>> + *
>> + * Returns: 0 on success and dest will contains the decrypted data
>> + */
>> +int kvm_sev_dbg_decrypt(uint8_t *dest, const uint8_t *src, uint32_t len);
>> +
>>  #endif
>> diff --git a/sev.c b/sev.c
>> index 055ed83..c7031d3 100644
>> --- a/sev.c
>> +++ b/sev.c
>> @@ -432,3 +432,26 @@ int kvm_sev_guest_measurement(uint8_t *out)
>>  
>>      return 0;
>>  }
>> +
>> +int kvm_sev_dbg_decrypt(uint8_t *dst, const uint8_t *src, uint32_t len)
>> +{
>> +    int ret;
>> +    struct kvm_sev_dbg_decrypt decrypt;
>> +    struct kvm_sev_issue_cmd input;
>> +
>> +    decrypt.src_addr = (unsigned long)src;
>> +    decrypt.dst_addr = (unsigned long)dst;
>> +    decrypt.length = len;
>> +
>> +    input.cmd = KVM_SEV_DBG_DECRYPT;
>> +    input.opaque = (unsigned long)&decrypt;
>> +    ret = kvm_vm_ioctl(kvm_state, KVM_SEV_ISSUE_CMD, &input);
>> +    if (ret) {
>> +        fprintf(stderr, "SEV: dbg_decrypt failed ret=%d(%#010x)\n",
>> +                ret, input.ret_code);
>> +        return 1;
>> +    }
>> +
>> +    DPRINTF("SEV: DBG_DECRYPT dst %p src %p sz %d\n", dst, src, len);
>> +    return 0;
>> +}
> 
>