[kvmtool] Reset all VCPUs before any entering run loops

[kvmtool] Reset all VCPUs before any entering run loops

[Keir Fraser]

VCPU threads may currently enter their run loops before all other
VCPUs have "reset" to an appropriate initial state.

Actually this normally works okay, but on pKVM-ARM the VM's Hyp state
(including boot VCPU's initial state) gets set up by the first VCPU
thread to call ioctl(KVM_RUN). This races boot VCPU thread's
intialisation of register state, and can result in the boot VCPU
starting execution at PC=0.

The simplest fix is to reset each VCPU when it is first created,
before the VCPU run threads are even created.

Signed-off-by: Keir Fraser <keirf@google.com>
---
 kvm-cpu.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kvm-cpu.c b/kvm-cpu.c
index 1c566b3..f66dcd0 100644
--- a/kvm-cpu.c
+++ b/kvm-cpu.c
@@ -154,8 +154,6 @@ int kvm_cpu__start(struct kvm_cpu *cpu)
 	signal(SIGKVMPAUSE, kvm_cpu_signal_handler);
 	signal(SIGKVMTASK, kvm_cpu_signal_handler);
 
-	kvm_cpu__reset_vcpu(cpu);
-
 	if (cpu->kvm->cfg.single_step)
 		kvm_cpu__enable_singlestep(cpu);
 
@@ -293,6 +291,7 @@ int kvm_cpu__init(struct kvm *kvm)
 			pr_err("unable to initialize KVM VCPU");
 			goto fail_alloc;
 		}
+		kvm_cpu__reset_vcpu(kvm->cpus[i]);
 	}
 
 	return 0;
-- 
2.47.1.545.g3c1d2e2a6a-goog

Re: [kvmtool] Reset all VCPUs before any entering run loops

[Will Deacon]

On Wed, 11 Dec 2024 09:45:14 +0000, Keir Fraser wrote:
> VCPU threads may currently enter their run loops before all other
> VCPUs have "reset" to an appropriate initial state.
> 
> Actually this normally works okay, but on pKVM-ARM the VM's Hyp state
> (including boot VCPU's initial state) gets set up by the first VCPU
> thread to call ioctl(KVM_RUN). This races boot VCPU thread's
> intialisation of register state, and can result in the boot VCPU
> starting execution at PC=0.
> 
> [...]

Applied to kvmtool (master), thanks!

[1/1] Reset all VCPUs before any entering run loops
      https://git.kernel.org/will/kvmtool/c/6d754d01fe2c

Cheers,
-- 
Will

https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev