Re: [PATCH v6 01/14] KVM: SVM: Add KVM_SEV SEND_START command

[Brijesh Singh]

On 4/2/20 2:43 PM, Venu Busireddy wrote:
> On 2020-04-02 14:17:26 -0500, Brijesh Singh wrote:
>> On 4/2/20 1:57 PM, Venu Busireddy wrote:
>> [snip]...
>>
>>>> The question is, how does a userspace know the session length ? One
>>>> method is you can precalculate a value based on your firmware version
>>>> and have userspace pass that, or another approach is set
>>>> params.session_len = 0 and query it from the FW. The FW spec allow to
>>>> query the length, please see the spec. In the qemu patches I choose
>>>> second approach. This is because session blob can change from one FW
>>>> version to another and I tried to avoid calculating or hardcoding the
>>>> length for a one version of the FW. You can certainly choose the first
>>>> method. We want to ensure that kernel interface works on the both cases.
>>> I like the fact that you have already implemented the functionality to
>>> facilitate the user space to obtain the session length from the firmware
>>> (by setting params.session_len to 0). However, I am trying to address
>>> the case where the user space sets the params.session_len to a size
>>> smaller than the size needed.
>>>
>>> Let me put it differently. Let us say that the session blob needs 128
>>> bytes, but the user space sets params.session_len to 16. That results
>>> in us allocating a buffer of 16 bytes, and set data->session_len to 16.
>>>
>>> What does the firmware do now?
>>>
>>> Does it copy 128 bytes into data->session_address, or, does it copy
>>> 16 bytes?
>>>
>>> If it copies 128 bytes, we most certainly will end up with a kernel crash.
>>>
>>> If it copies 16 bytes, then what does it set in data->session_len? 16,
>>> or 128? If 16, everything is good. If 128, we end up causing memory
>>> access violation for the user space.
>> My interpretation of the spec is, if user provided length is smaller
>> than the FW expected length then FW will reports an error with
>> data->session_len set to the expected length. In other words, it should
>> *not* copy anything into the session buffer in the event of failure.
> That is good, and expected behavior.
>
>> If FW is touching memory beyond what is specified in the session_len then
>> its FW bug and we can't do much from kernel.
> Agreed. But let us assume that the firmware is not touching memory that
> it is not supposed to.
>
>> Am I missing something ?
> I believe you are agreeing that if the session blob needs 128 bytes and
> user space sets params.session_len to 16, the firmware does not copy
> any data to data->session_address, and sets data->session_len to 128.
>
> Now, when we return, won't the user space try to access 128 bytes
> (params.session_len) of data in params.session_uaddr, and crash? Because,
> instead of returning an error that buffer is not large enough, we return
> the call successfully!


Ah, so the main issue is we should not be going to e_free on error. If
session_len is less than the expected len then FW will return an error.
In the case of an error we can skip copying the session_data into
userspace buffer but we still need to pass the session_len and policy
back to the userspace.

+	ret = sev_issue_cmd(kvm, SEV_CMD_SEND_START, data, &argp->error);
+
+	if (ret)
+		goto e_free;
+

If user space gets an error then it can decode it further to get
additional information (e.g buffer too small).

>
> That is why I was suggesting the following, which you seem to have
> missed.
>
>>> Perhaps, this can be dealt a little differently? Why not always call
>>> sev_issue_cmd(kvm, SEV_CMD_SEND_START, ...) with zeroed out data? Then,
>>> if the user space has set params.session_len to 0, we return with the
>>> needed params.session_len. Otherwise, we check if params.session_len is
>>> large enough, and if not, we return -EINVAL?
> Doesn't the above approach address all scenarios?
>