[RFT PATCH] x86/hyperv: Use __naked attribute to fix stackless C function

[Ard Biesheuvel <ardb+git@google.com>]

From: Ard Biesheuvel <ardb@kernel.org>

hv_crash_c_entry() is a C function that is entered without a stack,
and this is only allowed for functions that have the __naked attribute,
which informs the compiler that it must not emit the usual prologue and
epilogue or emit any other kind of instrumentation that relies on a
stack frame.

So split up the function, and set the __naked attribute on the initial
part that sets up the stack, GDT, IDT and other pieces that are needed
for ordinary C execution. Given that function calls are not permitted
either, use the existing long return coded in an asm() block to call the
second part of the function, which is an ordinary function that is
permitted to call other functions as usual.

Fixes: 94212d34618c ("x86/hyperv: Implement hypervisor RAM collection into vmcore")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
Build tested only.

Cc: Mukesh Rathor <mrathor@linux.microsoft.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Wei Liu <wei.liu@kernel.org>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Long Li <longli@microsoft.com>
Cc: Thomas Gleixner <tglx@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Uros Bizjak <ubizjak@gmail.com>
Cc: linux-hyperv@vger.kernel.org

 arch/x86/hyperv/hv_crash.c | 80 ++++++++++----------
 1 file changed, 42 insertions(+), 38 deletions(-)

diff --git a/arch/x86/hyperv/hv_crash.c b/arch/x86/hyperv/hv_crash.c
index a78e4fed5720..d77766e8d37e 100644
--- a/arch/x86/hyperv/hv_crash.c
+++ b/arch/x86/hyperv/hv_crash.c
@@ -107,14 +107,12 @@ static void __noreturn hv_panic_timeout_reboot(void)
 		cpu_relax();
 }
 
-/* This cannot be inlined as it needs stack */
-static noinline __noclone void hv_crash_restore_tss(void)
+static void hv_crash_restore_tss(void)
 {
 	load_TR_desc();
 }
 
-/* This cannot be inlined as it needs stack */
-static noinline void hv_crash_clear_kernpt(void)
+static void hv_crash_clear_kernpt(void)
 {
 	pgd_t *pgd;
 	p4d_t *p4d;
@@ -125,6 +123,25 @@ static noinline void hv_crash_clear_kernpt(void)
 	native_p4d_clear(p4d);
 }
 
+
+static void __noreturn hv_crash_handle(void)
+{
+	hv_crash_restore_tss();
+	hv_crash_clear_kernpt();
+
+	/* we are now fully in devirtualized normal kernel mode */
+	__crash_kexec(NULL);
+
+	hv_panic_timeout_reboot();
+}
+
+/*
+ * __naked functions do not permit function calls, not even to __always_inline
+ * functions that only contain asm() blocks themselves. So use a macro instead.
+ */
+#define hv_wrmsr(msr, val) \
+	asm("wrmsr" :: "c"(msr), "a"((u32)val), "d"((u32)(val >> 32)) : "memory")
+
 /*
  * This is the C entry point from the asm glue code after the disable hypercall.
  * We enter here in IA32-e long mode, ie, full 64bit mode running on kernel
@@ -133,49 +150,36 @@ static noinline void hv_crash_clear_kernpt(void)
  * available. We restore kernel GDT, and rest of the context, and continue
  * to kexec.
  */
-static asmlinkage void __noreturn hv_crash_c_entry(void)
+static void __naked hv_crash_c_entry(void)
 {
-	struct hv_crash_ctxt *ctxt = &hv_crash_ctxt;
-
 	/* first thing, restore kernel gdt */
-	native_load_gdt(&ctxt->gdtr);
+	asm volatile("lgdt %0" : : "m" (hv_crash_ctxt.gdtr));
 
-	asm volatile("movw %%ax, %%ss" : : "a"(ctxt->ss));
-	asm volatile("movq %0, %%rsp" : : "m"(ctxt->rsp));
+	asm volatile("movw %%ax, %%ss" : : "a"(hv_crash_ctxt.ss));
+	asm volatile("movq %0, %%rsp" : : "m"(hv_crash_ctxt.rsp));
 
-	asm volatile("movw %%ax, %%ds" : : "a"(ctxt->ds));
-	asm volatile("movw %%ax, %%es" : : "a"(ctxt->es));
-	asm volatile("movw %%ax, %%fs" : : "a"(ctxt->fs));
-	asm volatile("movw %%ax, %%gs" : : "a"(ctxt->gs));
+	asm volatile("movw %%ax, %%ds" : : "a"(hv_crash_ctxt.ds));
+	asm volatile("movw %%ax, %%es" : : "a"(hv_crash_ctxt.es));
+	asm volatile("movw %%ax, %%fs" : : "a"(hv_crash_ctxt.fs));
+	asm volatile("movw %%ax, %%gs" : : "a"(hv_crash_ctxt.gs));
 
-	native_wrmsrq(MSR_IA32_CR_PAT, ctxt->pat);
-	asm volatile("movq %0, %%cr0" : : "r"(ctxt->cr0));
+	hv_wrmsr(MSR_IA32_CR_PAT, hv_crash_ctxt.pat);
+	asm volatile("movq %0, %%cr0" : : "r"(hv_crash_ctxt.cr0));
 
-	asm volatile("movq %0, %%cr8" : : "r"(ctxt->cr8));
-	asm volatile("movq %0, %%cr4" : : "r"(ctxt->cr4));
-	asm volatile("movq %0, %%cr2" : : "r"(ctxt->cr4));
+	asm volatile("movq %0, %%cr8" : : "r"(hv_crash_ctxt.cr8));
+	asm volatile("movq %0, %%cr4" : : "r"(hv_crash_ctxt.cr4));
+	asm volatile("movq %0, %%cr2" : : "r"(hv_crash_ctxt.cr4));
 
-	native_load_idt(&ctxt->idtr);
-	native_wrmsrq(MSR_GS_BASE, ctxt->gsbase);
-	native_wrmsrq(MSR_EFER, ctxt->efer);
+	asm volatile("lidt %0" : : "m" (hv_crash_ctxt.idtr));
+	hv_wrmsr(MSR_GS_BASE, hv_crash_ctxt.gsbase);
+	hv_wrmsr(MSR_EFER, hv_crash_ctxt.efer);
 
 	/* restore the original kernel CS now via far return */
-	asm volatile("movzwq %0, %%rax\n\t"
-		     "pushq %%rax\n\t"
-		     "pushq $1f\n\t"
-		     "lretq\n\t"
-		     "1:nop\n\t" : : "m"(ctxt->cs) : "rax");
-
-	/* We are in asmlinkage without stack frame, hence make C function
-	 * calls which will buy stack frames.
-	 */
-	hv_crash_restore_tss();
-	hv_crash_clear_kernpt();
-
-	/* we are now fully in devirtualized normal kernel mode */
-	__crash_kexec(NULL);
-
-	hv_panic_timeout_reboot();
+	asm volatile("pushq	%q0		\n\t"
+		     "leaq	%c1(%%rip), %q0	\n\t"
+		     "pushq	%q0		\n\t"
+		     "lretq			\n\t"
+		     :: "a"(hv_crash_ctxt.cs), "i"(hv_crash_handle));
 }
 /* Tell gcc we are using lretq long jump in the above function intentionally */
 STACK_FRAME_NON_STANDARD(hv_crash_c_entry);
-- 
2.53.0.414.gf7e9f6c205-goog