Re: KCSAN + KVM = host reset

Qian Cai
2020-04-10T19:57:18Z

> On Apr 10, 2020, at 7:35 AM, Marco Elver <elver@google.com> wrote:
>
> On Fri, 10 Apr 2020 at 13:25, Qian Cai <cai@lca.pw> wrote:
>>
>>
>>
>>> On Apr 10, 2020, at 5:47 AM, Marco Elver <elver@google.com> wrote:
>>>
>>> That would contradict what you said about it working if KCSAN is
>>> "off". What kernel are you attempting to use in the VM?
>
> Ah, sorry this was a typo,
> s/working if KCSAN/not working if KCSAN/
>
>> Well, I said set KCSAN debugfs to “off” did not help, i.e., it will reset the host running kvm.sh. It is the vanilla ubuntu 18.04 kernel in VM.
>>
>> github.com/cailca/linux-mm/blob/master/kvm.sh
>
> So, if you say that CONFIG_KCSAN_INTERRUPT_WATCHER=n works, that
> contradicts it not working when KCSAN is "off". Because if KCSAN is
> off, it never sets up any watchpoints, and whether or not
> KCSAN_INTERRUPT_WATCHER is selected or not shouldn't matter. Does that
> make more sense?

Yes, you are right. CONFIG_KCSAN_INTERRUPT_WATCHER=n does not
make it work. It was a mistake when I tested it because there was a stale svm.o
leftover from the previous run, and then it will not trigger a rebuild (a bug?) when
only modify the Makefile to remove KCSAN_SANITIZE := n. Sorry for the misleading
information. I should be checking if svm.o was really recompiled in the first place.

Anyway, I’ll send a patch to add __no_kcsan for svm_vcpu_run() because I tried
to narrow down more with a kcsan_[disable|enable]_current() pair, but it does NOT
work even by enclosing the almost whole function below until Marcro has more ideas?

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 2be5bbae3a40..e58b2d5a575c 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3286,6 +3286,7 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
svm->vmcb->save.rsp = vcpu->arch.regs[VCPU_REGS_RSP];
svm->vmcb->save.rip = vcpu->arch.regs[VCPU_REGS_RIP];

+ kcsan_disable_current();
/*
* A vmexit emulation is required before the vcpu can be executed
* again.
@@ -3410,6 +3411,7 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
svm_handle_mce(svm);

mark_all_clean(svm->vmcb);
+ kcsan_enable_current();
}
STACK_FRAME_NON_STANDARD(svm_vcpu_run);