Re: CONFIG_KCOV causing crash in svm_vcpu_run()

[Eric Biggers]

Sorry, messed up address for KVM mailing list.  See message below.

On Sun, May 13, 2018 at 08:00:07PM -0700, Eric Biggers wrote:
> With CONFIG_KCOV=y and an AMD processor, running the following program crashes
> the kernel with no output (I'm testing in a VM, so it's using nested
> virtualization):
> 
> 	#include <fcntl.h>
> 	#include <linux/kvm.h>
> 	#include <sys/ioctl.h>
> 
> 	int main()
> 	{
> 		int dev, vm, cpu;
> 		char page[4096] __attribute__((aligned(4096))) = { 0 };
> 		struct kvm_userspace_memory_region memreg = {
> 			.memory_size = 4096,
> 			.userspace_addr = (unsigned long)page,
> 		};
> 		dev = open("/dev/kvm", O_RDONLY);
> 		vm = ioctl(dev, KVM_CREATE_VM, 0);
> 		cpu = ioctl(vm, KVM_CREATE_VCPU, 0);
> 		ioctl(vm, KVM_SET_USER_MEMORY_REGION, &memreg);
> 		ioctl(cpu, KVM_RUN, 0);
> 	}
> 
> It bisects down to commit b2ac58f90540e39 ("KVM/SVM: Allow direct access to
> MSR_IA32_SPEC_CTRL").  The bug is apparently that due to the new code for
> managing the SPEC_CTRL MSR, __sanitizer_cov_trace_pc() is being called from
> svm_vcpu_run() before the host's MSR_GS_BASE has been restored, which causes a
> crash somehow.  The following patch fixes it, though I don't know that it's the
> right solution; maybe KCOV should be disabled in the function instead, or maybe
> there's a more fundamental problem.  What do people think?
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 1fc05e428aba8..d35ef241e66d8 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -5652,6 +5652,15 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
>  #endif
>  		);
>  
> +#ifdef CONFIG_X86_64
> +	wrmsrl(MSR_GS_BASE, svm->host.gs_base);
> +#else
> +	loadsegment(fs, svm->host.fs);
> +#ifndef CONFIG_X86_32_LAZY_GS
> +	loadsegment(gs, svm->host.gs);
> +#endif
> +#endif
> +
>  	/*
>  	 * We do not use IBRS in the kernel. If this vCPU has used the
>  	 * SPEC_CTRL MSR it may have left it on; save the value and
> @@ -5676,15 +5685,6 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
>  	/* Eliminate branch target predictions from guest mode */
>  	vmexit_fill_RSB();
>  
> -#ifdef CONFIG_X86_64
> -	wrmsrl(MSR_GS_BASE, svm->host.gs_base);
> -#else
> -	loadsegment(fs, svm->host.fs);
> -#ifndef CONFIG_X86_32_LAZY_GS
> -	loadsegment(gs, svm->host.gs);
> -#endif
> -#endif
> -
>  	reload_tss(vcpu);
>  
>  	local_irq_disable();