linux-kernel - [PATCH 5/6] x86_64,hyperv: Use direct call to hypercall-page

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Message-ID: <20250414113754.435282530@infradead.org>
Date: Mon, 14 Apr 2025 13:11:45 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: x86@...nel.org
Cc: kys@...rosoft.com,
 haiyangz@...rosoft.com,
 wei.liu@...nel.org,
 decui@...rosoft.com,
 tglx@...utronix.de,
 mingo@...hat.com,
 bp@...en8.de,
 dave.hansen@...ux.intel.com,
 hpa@...or.com,
 peterz@...radead.org,
 jpoimboe@...nel.org,
 pawan.kumar.gupta@...ux.intel.com,
 seanjc@...gle.com,
 pbonzini@...hat.com,
 ardb@...nel.org,
 kees@...nel.org,
 Arnd Bergmann <arnd@...db.de>,
 gregkh@...uxfoundation.org,
 linux-hyperv@...r.kernel.org,
 linux-kernel@...r.kernel.org,
 kvm@...r.kernel.org,
 linux-efi@...r.kernel.org,
 samitolvanen@...gle.com,
 ojeda@...nel.org
Subject: [PATCH 5/6] x86_64,hyperv: Use direct call to hypercall-page

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@...radead.org>
---
 arch/x86/hyperv/hv_init.c |   62 +++++++++++++++++++++++-----------------------
 1 file changed, 31 insertions(+), 31 deletions(-)

--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -37,24 +37,42 @@
 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_pg_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)
+		      : "r" (__r8)
 		      : "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
 
@@ -349,7 +367,7 @@ 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);
@@ -375,7 +393,7 @@ 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;
 
 	/*
@@ -529,8 +547,8 @@ 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)
@@ -568,27 +586,9 @@ 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
@@ -658,7 +658,7 @@ void hyperv_cleanup(void)
 	 * let hypercall operations fail safely rather than
 	 * panic the kernel for using invalid hypercall page
 	 */
-	hv_hypercall_pg = NULL;
+	hv_set_hypercall_pg(NULL);
 
 	/* Reset the hypercall page */
 	hypercall_msr.as_uint64 = hv_get_msr(HV_X64_MSR_HYPERCALL);