This is a note to let you know that I've just added the patch titled
x86/hyperv: Disable IBT when hypercall page lacks ENDBR instruction
to the 6.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
x86-hyperv-disable-ibt-when-hypercall-page-lacks-endbr-instruction.patch
and it can be found in the queue-6.1 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From d5ace2a776442d80674eff9ed42e737f7dd95056 Mon Sep 17 00:00:00 2001
From: Michael Kelley <mikelley@xxxxxxxxxxxxx>
Date: Fri, 21 Jul 2023 21:51:16 -0700
Subject: x86/hyperv: Disable IBT when hypercall page lacks ENDBR instruction
From: Michael Kelley <mikelley@xxxxxxxxxxxxx>
commit d5ace2a776442d80674eff9ed42e737f7dd95056 upstream.
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>
Link: https://lore.kernel.org/r/1690001476-98594-1-git-send-email-mikelley@xxxxxxxxxxxxx
Signed-off-by: Wei Liu <wei.liu@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/x86/hyperv/hv_init.c | 21 +++++++++++++++++++++
1 file changed, 21 insertions(+)
--- 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>
@@ -468,6 +469,26 @@ 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
Patches currently in stable-queue which might be from mikelley@xxxxxxxxxxxxx are
queue-6.1/scsi-storvsc-limit-max_sectors-for-virtual-fibre-channel-devices.patch
queue-6.1/x86-hyperv-disable-ibt-when-hypercall-page-lacks-endbr-instruction.patch
