On Fri, Jan 24, 2025 at 04:10:45PM -0800, Sean Christopherson wrote: > On Fri, Jan 24, 2025, Keith Busch wrote: > > On Fri, Jan 24, 2025 at 12:07:24PM -0800, Sean Christopherson wrote: > > > This is broken. If the module param is toggled before the first KVM_RUN, KVM > > > will hit a NULL pointer deref due to trying to start a non-existent vhost task: > > > > > > BUG: kernel NULL pointer dereference, address: 0000000000000040 > > > #PF: supervisor read access in kernel mode > > > #PF: error_code(0x0000) - not-present page > > > PGD 0 P4D 0 > > > Oops: Oops: 0000 [#1] SMP > > > CPU: 16 UID: 0 PID: 1190 Comm: bash Not tainted 6.13.0-rc3-9bb02e874121-x86/xen_msr_fixes-vm #2382 > > > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015 > > > RIP: 0010:vhost_task_wake+0x5/0x10 > > > Call Trace: > > > <TASK> > > > set_nx_huge_pages+0xcc/0x1e0 [kvm] > > > > Thanks for pointing out this gap. It looks like we'd have to hold the > > kvm_lock in kvm_mmu_post_init_vm(), and add NULL checks in > > set_nx_huge_pages() and set_nx_huge_pages_recovery_param() to prevent > > the NULL deref. Is that okay? > > I don't _think_ we need to take kvm_lock. And I don't want to take kvm_lock, > because we're also trying to eliminate a (very theoretical) deadlock[1] due to > taking kvm_lock in the params helpers. > > There is a race that can happen with my proposed fix[2], but I'm not sure we care > enough to address it. If kvm_nx_huge_page_recovery_worker() runs before the params > are set, and the param setter processes the VM before nx_huge_page_recovery_thread > is set, then the worker could sleep for too long, relative to what userspace expects. > > I suppose if we care then we could fix that by taking kvm->arch.nx_once.mutex > when waking the task? I think we actually can do this without any additional locks. The only thing we need to ensure is that the vhost task sees the updated variable, and I think we can achieve that with appropriate memory barriers on the reads and writes.
