[PATCH v2 12/13] x86_64,hyperv: Use direct call to hypercall-page

Peter Zijlstra
2025-04-30T11:26:47Z

Instead of using an indirect call to the hypercall page, use a direct
call instead. This avoids all CFI problems, including the one where
the hypercall page doesn't have IBT on.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
arch/x86/hyperv/hv_init.c | 60 +++++++++++++++++++++++-----------------------
1 file changed, 30 insertions(+), 30 deletions(-)

--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -37,23 +37,41 @@
void *hv_hypercall_pg;

#ifdef CONFIG_X86_64
+static u64 __hv_hyperfail(u64 control, u64 param1, u64 param2)
+{
+ return U64_MAX;
+}
+
+DEFINE_STATIC_CALL(__hv_hypercall, __hv_hyperfail);
+
u64 hv_std_hypercall(u64 control, u64 param1, u64 param2)
{
u64 hv_status;

- if (!hv_hypercall_pg)
- return U64_MAX;
-
register u64 __r8 asm("r8") = param2;
- asm volatile (CALL_NOSPEC
+ asm volatile ("call " STATIC_CALL_TRAMP_STR(__hv_hypercall)
: "=a" (hv_status), ASM_CALL_CONSTRAINT,
"+c" (control), "+d" (param1), "+r" (__r8)
- : THUNK_TARGET(hv_hypercall_pg)
- : "cc", "memory", "r9", "r10", "r11");
+ : : "cc", "memory", "r9", "r10", "r11");

return hv_status;
}
+
+typedef u64 (*hv_hypercall_f)(u64 control, u64 param1, u64 param2);
+
+static inline void hv_set_hypercall_pg(void *ptr)
+{
+ hv_hypercall_pg = ptr;
+
+ if (!ptr)
+ ptr = &__hv_hyperfail;
+ static_call_update(__hv_hypercall, (hv_hypercall_f)ptr);
+}
#else
+static inline void hv_set_hypercall_pg(void *ptr)
+{
+ hv_hypercall_pg = ptr;
+}
EXPORT_SYMBOL_GPL(hv_hypercall_pg);
#endif

@@ -348,7 +366,7 @@ static int hv_suspend(void)
* pointer is restored on resume.
*/
hv_hypercall_pg_saved = hv_hypercall_pg;
- hv_hypercall_pg = NULL;
+ hv_set_hypercall_pg(NULL);

/* Disable the hypercall page in the hypervisor */
rdmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
@@ -374,7 +392,7 @@ static void hv_resume(void)
vmalloc_to_pfn(hv_hypercall_pg_saved);
wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);

- hv_hypercall_pg = hv_hypercall_pg_saved;
+ hv_set_hypercall_pg(hv_hypercall_pg_saved);
hv_hypercall_pg_saved = NULL;

/*
@@ -528,8 +546,8 @@ void __init hyperv_init(void)
if (hv_isolation_type_tdx() && !ms_hyperv.paravisor_present)
goto skip_hypercall_pg_init;

- hv_hypercall_pg = __vmalloc_node_range(PAGE_SIZE, 1, VMALLOC_START,
- VMALLOC_END, GFP_KERNEL, PAGE_KERNEL_ROX,
+ hv_hypercall_pg = __vmalloc_node_range(PAGE_SIZE, 1, MODULES_VADDR,
+ MODULES_END, GFP_KERNEL, PAGE_KERNEL_ROX,
VM_FLUSH_RESET_PERMS, NUMA_NO_NODE,
__builtin_return_address(0));
if (hv_hypercall_pg == NULL)
@@ -567,27 +585,9 @@ void __init hyperv_init(void)
wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
}

-skip_hypercall_pg_init:
- /*
- * 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("Disabling IBT because of Hyper-V bug\n");
- }
-#endif
+ hv_set_hypercall_pg(hv_hypercall_pg);

+skip_hypercall_pg_init:
/*
* hyperv_init() is called before LAPIC is initialized: see
* apic_intr_mode_init() -> x86_platform.apic_post_init() and