On Thu, Aug 19, 2021, Marc Orr wrote: > On Thu, Aug 19, 2021 at 3:58 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > > > On Thu, Aug 19, 2021, Peter Gonda wrote: > > > Marc I think that only having the spin lock could result in > > > deadlocking. If userspace double migrated 2 VMs, A and B for > > > discussion, A could grab VM_A.spin_lock then VM_A.kvm_mutex. Meanwhile > > > B could grab VM_B.spin_lock and VM_B.kvm_mutex. Then A attempts to > > > grab VM_B.spin_lock and we have a deadlock. If the same happens with > > > the proposed scheme when A attempts to lock B, VM_B.spin_lock will be > > > open but the bool will mark the VM under migration so A will unlock > > > and bail. Sean originally proposed a global spin lock but I thought a > > > per kvm_sev_info struct would also be safe. > > > > Close. The issue is taking kvm->lock from both VM_A and VM_B. If userspace > > double migrates we'll end up with lock ordering A->B and B-A, so we need a way > > to guarantee one of those wins. My proposed solution is to use a flag as a sort > > of one-off "try lock" to detect a mean userspace. > > Got it now. Thanks to you both, for the explanation. By the way, just > to make sure I completely follow, I assume that if a "double > migration" occurs, then user space is mis-behaving -- correct? Yep. > But presumably, we need to reason about how to respond to such mis-behavior > so that buggy or malicious user-space code cannot stumble over/exploit this > scenario? That's what the anti-deadlock flag is for. :-) With that in place, there's no meaningful difference between say a bad userspace doing double migrate and a bad userspace migrating from garbage, e.g. passing in a bogus fd.
