Re: [PATCH v2 1/1] x86/hyperv: Disable IBT when hypercall page lacks ENDBR instruction

[Peter Zijlstra]

On Fri, Jul 21, 2023 at 09:51:16PM -0700, Michael Kelley wrote:
> On hardware that supports Indirect Branch Tracking (IBT), Hyper-V VMs
> with ConfigVersion 9.3 or later support IBT in the guest. However,
> current versions of Hyper-V have a bug in that there's not an ENDBR64
> instruction at the beginning of the hypercall page. Since hypercalls are
> made with an indirect call to the hypercall page, all hypercall attempts
> fail with an exception and Linux panics.
> 
> A Hyper-V fix is in progress to add ENDBR64. But guard against the Linux
> panic by clearing X86_FEATURE_IBT if the hypercall page doesn't start
> with ENDBR. The VM will boot and run without IBT.
> 
> If future Linux 32-bit kernels were to support IBT, additional hypercall
> page hackery would be needed to make IBT work for such kernels in a
> Hyper-V VM.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

> ---
> 
> Changes in v2:
> * Use pr_warn() instead of pr_info() [Peter Zijlstra]
> 
>  arch/x86/hyperv/hv_init.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> index 6c04b52..5cbee24 100644
> --- a/arch/x86/hyperv/hv_init.c
> +++ b/arch/x86/hyperv/hv_init.c
> @@ -14,6 +14,7 @@
>  #include <asm/apic.h>
>  #include <asm/desc.h>
>  #include <asm/sev.h>
> +#include <asm/ibt.h>
>  #include <asm/hypervisor.h>
>  #include <asm/hyperv-tlfs.h>
>  #include <asm/mshyperv.h>
> @@ -472,6 +473,26 @@ void __init hyperv_init(void)
>  	}
>  
>  	/*
> +	 * Some versions of Hyper-V that provide IBT in guest VMs have a bug
> +	 * in that there's no ENDBR64 instruction at the entry to the
> +	 * hypercall page. Because hypercalls are invoked via an indirect call
> +	 * to the hypercall page, all hypercall attempts fail when IBT is
> +	 * enabled, and Linux panics. For such buggy versions, disable IBT.
> +	 *
> +	 * Fixed versions of Hyper-V always provide ENDBR64 on the hypercall
> +	 * page, so if future Linux kernel versions enable IBT for 32-bit
> +	 * builds, additional hypercall page hackery will be required here
> +	 * to provide an ENDBR32.
> +	 */
> +#ifdef CONFIG_X86_KERNEL_IBT
> +	if (cpu_feature_enabled(X86_FEATURE_IBT) &&
> +	    *(u32 *)hv_hypercall_pg != gen_endbr()) {
> +		setup_clear_cpu_cap(X86_FEATURE_IBT);
> +		pr_warn("Hyper-V: Disabling IBT because of Hyper-V bug\n");
> +	}
> +#endif
> +
> +	/*
>  	 * hyperv_init() is called before LAPIC is initialized: see
>  	 * apic_intr_mode_init() -> x86_platform.apic_post_init() and
>  	 * apic_bsp_setup() -> setup_local_APIC(). The direct-mode STIMER
> -- 
> 1.8.3.1
>