KVM: nSVM: Remove a user-triggerable WARN on nested_svm_load_cr3() succeeding

In the Linux kernel, the following vulnerability has been resolved:

KVM: nSVM: Remove a user-triggerable WARN on nested_svm_load_cr3() succeeding

Drop the WARN in svm_set_nested_state() on nested_svm_load_cr3() failing

as it is trivially easy to trigger from userspace by modifying CPUID after

loading CR3. E.g. modifying the state restoration selftest like so:

--- tools/testing/selftests/kvm/x86/state_test.c

+++ tools/testing/selftests/kvm/x86/state_test.c

@@ -280,7 +280,16 @@ int main(int argc, char *argv[])

/* Restore state in a new VM. */

vcpu = vm_recreate_with_one_vcpu(vm);

- vcpu_load_state(vcpu, state);

+

+ if (stage == 4) {

+ state->sregs.cr3 = BIT(44);

+ vcpu_load_state(vcpu, state);

+

+ vcpu_set_cpuid_property(vcpu, X86_PROPERTY_MAX_PHY_ADDR, 36);

+ __vcpu_nested_state_set(vcpu, &state->nested);

+ } else {

+ vcpu_load_state(vcpu, state);

+ }

/*

* Restore XSAVE state in a dummy vCPU, first without doing

generates:

WARNING: CPU: 30 PID: 938 at arch/x86/kvm/svm/nested.c:1877 svm_set_nested_state+0x34a/0x360 [kvm_amd]

Modules linked in: kvm_amd kvm irqbypass [last unloaded: kvm]

CPU: 30 UID: 1000 PID: 938 Comm: state_test Tainted: G W 6.18.0-rc7-58e10b63777d-next-vm

Tainted: [W]=WARN

Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015

RIP: 0010:svm_set_nested_state+0x34a/0x360 [kvm_amd]

Call Trace:

<TASK>

kvm_arch_vcpu_ioctl+0xf33/0x1700 [kvm]

kvm_vcpu_ioctl+0x4e6/0x8f0 [kvm]

__x64_sys_ioctl+0x8f/0xd0

do_syscall_64+0x61/0xad0

entry_SYSCALL_64_after_hwframe+0x4b/0x53

Simply delete the WARN instead of trying to prevent userspace from shoving

"illegal" state into CR3. For better or worse, KVM's ABI allows userspace

to set CPUID after SREGS, and vice versa, and KVM is very permissive when

it comes to guest CPUID. I.e. attempting to enforce the virtual CPU model

when setting CPUID could break userspace. Given that the WARN doesn't

provide any meaningful protection for KVM or benefit for userspace, simply

drop it even though the odds of breaking userspace are minuscule.

Opportunistically delete a spurious newline.