Message-ID: <CAMkAt6r5MJq0rGYg7MAqm83Xp4mBADSKtQxV=i2_OFuQnDd5Yg@mail.gmail.com>
Date:   Fri, 12 Nov 2021 09:55:14 -0700
From:   Peter Gonda <pgonda@...gle.com>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     Sean Christopherson <seanjc@...gle.com>, Thomas.Lendacky@....com,
        David Rientjes <rientjes@...gle.com>,
        Marc Orr <marcorr@...gle.com>, Joerg Roedel <jroedel@...e.de>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        John Allen <john.allen@....com>,
        "David S. Miller" <davem@...emloft.net>,
        Paolo Bonzini <pbonzini@...hat.com>,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 4/4] crypto: ccp - Add SEV_INIT_EX support

On Wed, Nov 10, 2021 at 8:32 AM Peter Gonda <pgonda@...gle.com> wrote:
>
> On Tue, Nov 9, 2021 at 3:20 PM Brijesh Singh <brijesh.singh@....com> wrote:
> >
> >
> >
> > On 11/9/21 2:46 PM, Peter Gonda wrote:
> > > On Tue, Nov 9, 2021 at 1:26 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > >>
> > >> On Tue, Nov 09, 2021, Peter Gonda wrote:
> > >>> On Tue, Nov 9, 2021 at 10:21 AM Sean Christopherson <seanjc@...gle.com> wrote:
> > >>>> There's no need for this to be a function pointer, and the duplicate code can be
> > >>>> consolidated.
> > >>>>
> > >>>> static int sev_do_init_locked(int cmd, void *data, int *error)
> > >>>> {
> > >>>>          if (sev_es_tmr) {
> > >>>>                  /*
> > >>>>                   * Do not include the encryption mask on the physical
> > >>>>                   * address of the TMR (firmware should clear it anyway).
> > >>>>                   */
> > >>>>                  data.flags |= SEV_INIT_FLAGS_SEV_ES;
> > >>>>                  data.tmr_address = __pa(sev_es_tmr);
> > >>>>                  data.tmr_len = SEV_ES_TMR_SIZE;
> > >>>>          }
> > >>>>          return __sev_do_cmd_locked(SEV_CMD_INIT, &data, error);
> > >>>> }
> > >>>>
> > >>>> static int __sev_init_locked(int *error)
> > >>>> {
> > >>>>          struct sev_data_init data;
> > >>>>
> > >>>>          memset(&data, 0, sizeof(data));
> > >>>>          return sev_do_init_locked(cmd, &data, error);
> > >>>> }
> > >>>>
> > >>>> static int __sev_init_ex_locked(int *error)
> > >>>> {
> > >>>>          struct sev_data_init_ex data;
> > >>>>
> > >>>>          memset(&data, 0, sizeof(data));
> > >>>>          data.length = sizeof(data);
> > >>>>          data.nv_address = __psp_pa(sev_init_ex_nv_address);
> > >>>>          data.nv_len = NV_LENGTH;
> > >>>>          return sev_do_init_locked(SEV_CMD_INIT_EX, &data, error);
> > >>>> }
> > >>>
> > >>> I am missing how this removes the duplication of the retry code,
> > >>> parameter checking, and other error checking code.. With what you have
> > >>> typed out I would assume I still need to function pointer between
> > >>> __sev_init_ex_locked and __sev_init_locked. Can you please elaborate
> > >>> here?
> > >>
> > >> Hmm.  Ah, I got distracted between the original thought, the realization that
> > >> the two commands used different structs, and typing up the above.
> > >>
> > >>> Also is there some reason the function pointer is not acceptable?
> > >>
> > >> It's not unacceptable, it would just be nice to avoid, assuming the alternative
> > >> is cleaner.  But I don't think any alternative is cleaner, since as you pointed
> > >> out the above is a half-baked thought.
> > >
> > > OK I'll leave as is.
> > >
> > >>
> > >>>>> +     rc = init_function(error);
> > >>>>>        if (rc && *error == SEV_RET_SECURE_DATA_INVALID) {
> > >>>>>                /*
> > >>>>>                 * INIT command returned an integrity check failure
> > >>>>> @@ -286,8 +423,8 @@ static int __sev_platform_init_locked(int *error)
> > >>>>>                 * failed and persistent state has been erased.
> > >>>>>                 * Retrying INIT command here should succeed.
> > >>>>>                 */
> > >>>>> -             dev_dbg(sev->dev, "SEV: retrying INIT command");
> > >>>>> -             rc = __sev_do_cmd_locked(SEV_CMD_INIT, &data, error);
> > >>>>> +             dev_notice(sev->dev, "SEV: retrying INIT command");
> > >>>>> +             rc = init_function(error);
> > >>>>
> > >>>> The above comment says "persistent state has been erased", but __sev_do_cmd_locked()
> > >>>> only writes back to the file if a relevant command was successful, which means
> > >>>> that rereading the userspace file in __sev_init_ex_locked() will retry INIT_EX
> > >>>> with the same garbage data.
> > >>>
> > >>> Ack my mistake, that comment is stale. I will update it so its correct
> > >>> for the INIT and INIT_EX flows.
> > >>>>
> > >>>> IMO, the behavior should be to read the file on load and then use the kernel buffer
> > >>>> without ever reloading (unless this is built as a module and is unloaded and reloaded).
> > >>>> The writeback then becomes opportunistic in the sense that if it fails for some reason,
> > >>>> the kernel's internal state isn't blasted away.
> > >>>
> > >>> One issue here is that the file read can fail on load so we use the
> > >>> late retry to guarantee we can read the file.
> > >>
> > >> But why continue loading if reading the file fails on load?
> > >>
> > >>> The other point seems like preference. Users may wish to shutdown the PSP FW,
> > >>> load a new file, and INIT_EX again with that new data. Why should we preclude
> > >>> them from that functionality?
> > >>
> > >> I don't think we should preclude that functionality, but it needs to be explicitly
> > >> tied to a userspace action, e.g. either on module load or on writing the param to
> > >> change the path.  If the latter is allowed, then it needs to be denied if the PSP
> > >> is initialized, otherwise the kernel will be in a non-coherent state and AFAICT
> > >> userspace will have a heck of a time even understanding what state has been used
> > >> to initialize the PSP.
> > >
> > > If this driver is builtin the filesystem will be unavailable during
> > > __init. Using the existing retries already built into
> > > sev_platform_init() also the file to be read once userspace is
> > > running, meaning the file system is usable. As I tried to explain in
> > > the commit message. We could remove the sev_platform_init call during
> > > sev_pci_init since this only actually needs to be initialized when the
> > > first command requiring it is issues (either reading some keys/certs
> > > from the PSP or launching an SEV guest). Then userspace in both the
> > > builtin and module usage would know running one of those commands
> > > cause the file to be read for PSP usage. Tom any thoughts on this?
> > >
> >
> > One thing to note is that if we do the INIT on the first command then
> > the first guest launch will take a longer. The init command is not
> > cheap (especially with the SNP, it may take a longer because it has to
> > do all those RMP setup etc). IIRC, in my early SEV series in I was doing
> > the INIT during the first command execution and based on the
> > recommendation moved to do the init on probe.
> >
> > Should we add a module param to control whether to do INIT on probe or
> > delay until the first command ?
>
> Thats a good point Brijesh. I've only been testing this with SEV and
> ES so haven't noticed that long setup time. I like the idea of a
> module parameter to decide when to INIT, that should satisfy Sean's
> concern that the user doesn't know when the INIT_EX file would be read
> and that there is extra retry code (duplicated between sev_pci_init
> and all the PSP commands). I'll get started on that.

I need a little guidance on how to proceed with this. Should I have
the new module parameter 'psp_init_on_probe' just disable PSP init on
module init if false. Or should it also disable PSP init during
command flow if it's true?

I was thinking I should just have 'psp_init_on_probe' default to true,
and if false it stops the PSP init during sev_pci_init(). If I add the
second change that seems like it changes the ABI. Thoughts?

>
> >
> > -Brijesh

Powered by blists - more mailing lists