On Thu, Aug 26, 2021 at 05:57:08PM -0700, Sean Christopherson wrote: > Use a per-CPU pointer to track perf's guest callbacks so that KVM can set > the callbacks more precisely and avoid a lurking NULL pointer dereference. I'm completely failing to see how per-cpu helps anything here... > On x86, KVM supports being built as a module and thus can be unloaded. > And because the shared callbacks are referenced from IRQ/NMI context, > unloading KVM can run concurrently with perf, and thus all of perf's > checks for a NULL perf_guest_cbs are flawed as perf_guest_cbs could be > nullified between the check and dereference. No longer allowing KVM to be a module would be *AWESOME*. I detest how much we have to export for KVM :/ Still, what stops KVM from doing a coherent unreg? Even the static_call() proposed in the other patch, unreg can do static_call_update() + synchronize_rcu() to ensure everybody sees the updated pointer (would require a quick audit to see all users are with preempt disabled, but I think your using per-cpu here already imposes the same).
