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

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

[Michael Kelley]

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@vger.kernel.org
Signed-off-by: Michael Kelley <mikelley@microsoft.com>
---

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

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

[Wei Liu]

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@vger.kernel.org
> Signed-off-by: Michael Kelley <mikelley@microsoft.com>

Applied to hyperv-fixes. Thanks.

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@vger.kernel.org
> Signed-off-by: Michael Kelley <mikelley@microsoft.com>

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

> ---
> 
> 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
>