On Thu, Dec 19, 2024 at 01:37:18PM +0000, Daniel P. Berrangé wrote: > On Thu, Dec 19, 2024 at 07:16:01AM -0600, Michael Roth wrote: > > On Thu, Dec 19, 2024 at 08:13:44AM +0000, Daniel P. Berrangé wrote: > > > 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.r...@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. > > > > VCEK endorsement keys can change as a result of SNP firmware updates, > > which can be done while SNP guests are running and are often done in such > > a way to patch bugs/security holes. VLEK endorsement keys can similarly be > > updated on a live host. Both these sorts of interactions cannot be made > > compatible with bundling certificates with attestation reports without some > > orchestration in place to keep them atomic relative to the endorsement > > key being used by firmware to sign attestation reports. Every CSP > > implementing this will need to solve it in some way, and I'm sure some > > will handle all this completely differently. But it will make > > interoperative management/tooling a mess, and having a reference > > implementation based around something common will make it easier to > > steer CSPs to that common solution and give management tools authors > > *some* reference approach to target rather than expecting to retrofit > > some custom solution on top. > > > > With these patches, you can update firmware and endorsement keys while > > SNP guests are running, but it requires write locks on any active > > certificates as defined here and in the kernel, and doing certificate > > file updates in place while that write lock is still held. I don't really > > think that's over-engineered. I think it's surprisingly simple given the > > potential complexity of the above-mentioned requirements. > > > > But yes if management tries to unlink certificates while SNP guests are > > running, all bets are off. But at that point they are not cooperating > > with kernel/QEMU, and we don't need to care about that. And if they > > really do need to blow away certificates for a complete re-install > > or data-wipe or whatever, at that point they'd just need to ensure > > they stop all their SNP guests first. > > IMHO we msut consider unlink() to be a valid thing, because the right > way for apps to perform crash safe atomic updates of existing files, > is to use rename() from a temporary file, and the rename() in has an > implicit unlink as part of its operation. ie apps would be doing: > > fd = open("foo.tmp") > write(fd, ...) > fsync(fd) > close(fd) > rename("foo.tmp", "foo")
If we still want to allow for this rather than enforcing in-place update, one alternative would be to just allow a separate lock file to be specified rather than locking the certificate file itself. That would provide a bit more flexibility. I can update the QEMU implementation to take -certs-lock-file in addition to -certs-file so they can be specified separately. And if -certs-lock-file is not specified then QEMU will just assume management handles things different or has agreed to not do endorsement key updates while SNP guests are running. I think we'd considered something like that originally but the thinking was that locking the certs themselves was more organic in terms of an "obvious"/natural solution. But it does end up being a bit more inflexible WRT how libraries/etc. might manage file updates underneath the covers, so maybe a lock file is the better approach after all. -Mike > > That final rename operation will have the same effect on the locks > as unlink(). To cope with this anything doing locking has to run > in a loop comparing the inode either side of acquiring the lock > > TLDR: if we're going to do locking in QEMU, it needs to be done > robustly. > > 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 :| >
