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

[Venu Busireddy]

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!

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?