On Thu, Jul 20, 2023 at 01:33:57PM -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.
Whoops :/
> 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.
There are currently no plans to add IBT support to 32bit.
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
> ---
> 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_info("Hyper-V: Disabling IBT because of Hyper-V bug\n");
> + }
> +#endif
pr_warn() perhaps?
Other than that, this seems fairly straight forward. One thing I
wondered about; wouldn't it be possible to re-write the indirect
hypercall thingies to a direct call? I mean, once we have the hypercall
page mapped, the address is known right?
