On Fri, Apr 26, 2024, Michael Roth wrote: > On Fri, Apr 26, 2024 at 05:10:10PM -0700, Sean Christopherson wrote: > > e.g. put the cert in a directory along with a lock. Actually, IIUC, there doesn't > > even need to be a separate lock file. I know very little about userspace programming, > > but common sense and a quick search tells me that file locks are a solved problem. > > > > E.g. it took me ~5 minutes of Googling to come up with this, which AFAICT does > > exactly what you want. > > > > touch ~/vlek.cert > > ( > > flock -e 200 > > echo "Locked the cert, sleeping for 10 seconds" > > sleep 10 > > echo "Igor, it's alive!!!!!!" > > ) 200< vlek.cert > > > > touch ~/vlek.cert > > ( > > flock -s 201 > > echo "Got me a shared lock, no updates for you!" > > ) 201< vlek.cert > > > > Hmm... I did completely miss this option. But I think there are still some > issues here. IIUC you're suggesting (for example): > > "Management": > a) writelock vlek.cert > b) perform SNP_LOAD_VLEK and update vlek.cert contents > c) unlock vlek.cert > > "QEMU": > a) readlock vlek.cert > b) copy cert into guest buffer > c) unlock vlek.cert > > The issue is that after "QEMU" unlocks and return the cert to KVM we'll > have: > > "KVM" > a) return from EXT_GUEST_REQ exit to userspace > b) issue the attestation report to firmware > c) return the attestation report and cert to the guest > > Between a) and b), "Management" can complete another entire update, but > the cert that it passes back to the guest will be stale relative to the > key used to sign the attestation report. I was thinking userspace would hold the lock across SEV_CMD_SNP_GUEST_REQUEST. QEMU: a) readlock vlek.cert b) copy cert into guest buffer c) set kvm_run->immediate_exit d) invoke KVM_RUN e) KVM sends SEV_CMD_SNP_GUEST_REQUEST to PSP f) KVM exits to userspace with -EINTR g) unlock vlek.cert h) invoke KVM_RUN (resume the guest) > If we need to take more time to explore other options it's not > absolutely necessary to have the kernel solve this now. But every userspace > will need to solve it in some way so it seemed like it might be nice to > have a simple reference implementation to start with. Shoving something into the kernel is not a "reference implementation", especially not when it impacts the ABI of multiple subsystems.