Re: [PATCH v1 3/3] i386/sev: Add KVM_EXIT_SNP_REQ_CERTS support for certificate-fetching

[Daniel P . Berrangé]

On Wed, Dec 18, 2024 at 04:29:51PM -0600, Michael Roth wrote:
> On Wed, Dec 18, 2024 at 05:50:52PM +0000, Daniel P. Berrangé wrote:
> > On Wed, Dec 18, 2024 at 09:49:39AM -0600, Michael Roth wrote:
> > > The GHCB specification[1] defines a VMGEXIT-based Guest Request
> > > hypercall to allow an SNP guest to issue encrypted requests directly to
> > > SNP firmware to do things like query the attestation report for the
> > > guest. These are generally handled purely in the kernel.
> > > 
> > > In some some cases, it's useful for the host to be able to additionally
> > > supply the certificate chain for the signing key that SNP firmware uses
> > > to sign these attestation reports. To allow for this, the GHCB
> > > specification defines an Extended Guest Request where this certificate
> > > data can be provided in a special format described in the GHCB spec.
> > > This certificate data may be global or guest-specific depending on how
> > > the guest was configured. Rather than providing interfaces to manage
> > > these within the kernel, KVM provides a new KVM_EXIT_SNP_REQ_CERTS exit
> > > to request the certificate contents from userspace. Implement support
> > > for that here.
> > > 
> > > To synchronize delivery of the certificates to the guest in a way where
> > > they will not be rendered invalid by updates to SNP firmware or
> > > attestation singing/endorsement keys by management tools outside the
> > > purview of QEMU, it is expected by users of KVM_EXIT_SNP_REQ_CERTS to
> > > obtain a shared/read lock on the certificate file prior to delivering
> > > them back to KVM. Only after this will the attestation report be
> > > retrieved from firmware and bundled with the certificate data, so QEMU
> > > must continue to hold the file lock until KVM confirms that the
> > > attestation report has been retrieved/bundled. This confirmation is done
> > > by way of the kvm_immediate_exit callback infrastructure that was
> > > introduced in a previous patch.
> > > 
> > > [1] "Guest Hypervisor Communication Block (GHCB) Standardization",
> > >     https://www.amd.com/en/developer/sev.html
> > > 
> > > Signed-off-by: Michael Roth <michael.roth@amd.com>
> > > ---
> > >  qapi/qom.json                 |  23 +++-
> > >  target/i386/kvm/kvm.c         |  10 ++
> > >  target/i386/sev-sysemu-stub.c |   5 +
> > >  target/i386/sev.c             | 249 ++++++++++++++++++++++++++++++++++
> > >  target/i386/sev.h             |   2 +
> > >  5 files changed, 288 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/qapi/qom.json b/qapi/qom.json
> > > index 28ce24cd8d..6eaf0e7721 100644
> > > --- a/qapi/qom.json
> > > +++ b/qapi/qom.json
> > > @@ -1034,6 +1034,25 @@
> > >  #     firmware.  Set this to true to disable the use of VCEK.
> > >  #     (default: false) (since: 9.1)
> > >  #
> > > +# @certs-path: Path to certificate data that can be passed to guests via
> > > +#              SNP Extended Guest Requests. File should be in the format
> > > +#              described in the GHCB specification. (default: none)
> > > +#              (since: 10.0)
> > 
> > Can we document the required format here explicitly, rather than expecting
> > users to go searching for specs which are often practically impossible
> > to find, and even harder to read & interpret ?
> 
> It'll be difficult to summarize in a way that will be self-reliant,
> since knowing the certificate format is not sufficient to make sure
> it coincides with the endorsement key being used by firmware. So I can't
> promise to completely reduce reliance on external specs, but at least
> give a better of the format and where those external specs will come
> into play in filling out the data.
> 
> If it needs to be at least somewhat self-sufficient then that might
> warrant a separate document in docs/system/i386/amd-memory-encryption.rst
> or somewhere thereabouts that summarizes the whole attestation flow and
> how certificates tie into that.
> 
> Any preferences?
> 
> > 
> > > diff --git a/target/i386/sev.c b/target/i386/sev.c
> > > index 1a4eb1ada6..2c41bdbccf 100644
> > > --- a/target/i386/sev.c
> > > +++ b/target/i386/sev.c
> > > @@ -157,6 +157,9 @@ struct SevSnpGuestState {
> > >      char *id_auth_base64;
> > >      uint8_t *id_auth;
> > >      char *host_data;
> > > +    char *certs_path;
> > > +    int certs_fd;
> > > +    uint32_t certs_timeout;
> > >  
> > >      struct kvm_sev_snp_launch_start kvm_start_conf;
> > >      struct kvm_sev_snp_launch_finish kvm_finish_conf;
> > > @@ -1355,6 +1358,215 @@ sev_snp_launch_finish(SevCommonState *sev_common)
> > >      }
> > >  }
> > >  
> > > +static int open_certs_locked(SevSnpGuestState *sev_snp_guest)
> > > +{
> > > +    int fd, ret;
> > > +
> > > +    if (sev_snp_guest->certs_fd != -1) {
> > > +        return 0;
> > > +    }
> > > +
> > > +    fd = qemu_open(sev_snp_guest->certs_path, O_RDONLY, NULL);
> > > +    if (fd == -1) {
> > > +        error_report("Unable to open certificate blob at path %s, ret 
> > > %d",
> > > +                     sev_snp_guest->certs_path, fd);
> > > +        return fd;
> > > +    }
> > > +
> > > +    ret = qemu_lock_fd(fd, 0, 0, false);
> > > +    if (ret == -EAGAIN || ret == -EACCES) {
> > > +        ret = -EAGAIN;
> > > +        goto out_close;
> > > +    } else if (ret) {
> > > +        goto out_close;
> > > +    }
> > 
> > This locking scheme is likely unsafe. Consider this sequence
> > 
> >   * QEMU runs qemu_open(path)
> >   * External mgmt app runs unlink(path)
> >   * External mgmt app runs open(path)
> >   * External mgmt app runs lock(fd)
> >   * QEMU runs qemu_lock_fd(fd)
> > 
> > QEMU has successfully acquired a lock on an FD that corresponds to a
> > deleted file, not the current existing file.
> > 
> > Avoiding this problem requires either that the external mgmt app agrees
> > to *NEVER* unlink() the files under any circumstance, or for QEMU to
> > run its open + lock logic in a loop, checking 'stat' and 'fstat' before
> > opening and after locking, in order to detect a replaced file from its
> > changed inode.
> > 
> > I'm not inclined to rely on mgmt apps never unlink()ing as that's to
> > easy to mess up IMHO.
> 
> Yah I went into more detail in my response to Markus, but long story
> short is that we are assuming mgmt is cooperative in this case, and
> so as you mentioned, it would never unlink files while SNP guests are
> running, but instead take an exclusive lock on them and update them in
> place with the understanding that doing anything otherwise would open
> a race window where guests might get stale certificates.

If there's an expectation & requirement that no SNP guests are running,
then IMHO this whole thing is just over-engineered. Just remove all this
locking code entirely, and document that none of this must be changed
while QEMU is running - which is a common requirement for a great many
things on the host.

With regards,
Daniel
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