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 <[email protected]> > > > --- > > > 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 -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
