[PATCH 1/2] x86/cfi: Fix ret_from_fork indirect calls

[Peter Zijlstra]

The ret_from_fork stub does an indirect call to the kthread function,
but only knows about Retpolines. Instead of making the asm more
complicated, punt to C and let the compiler figure it out.

Specifically, this makes it a proper kCFI indirect call when needed (in
fact, it is nearly impossible to code a kCFI indirect call in asm).

This was the only callsite that was still calling func()+0 on regular
indirect functions.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 arch/x86/entry/entry_64.S        |    6 ++++--
 arch/x86/include/asm/switch_to.h |    2 ++
 arch/x86/kernel/process_64.c     |    5 +++++
 3 files changed, 11 insertions(+), 2 deletions(-)

--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -304,8 +304,10 @@ SYM_CODE_START_NOALIGN(ret_from_fork)
 1:
 	/* kernel thread */
 	UNWIND_HINT_END_OF_STACK
-	movq	%r12, %rdi
-	CALL_NOSPEC rbx
+	movq	%rbx, %rdi
+	movq	%r12, %rsi
+	call	kthread_from_fork
+
 	/*
 	 * A kernel thread is allowed to return here after successfully
 	 * calling kernel_execve().  Exit to userspace to complete the execve()
--- a/arch/x86/include/asm/switch_to.h
+++ b/arch/x86/include/asm/switch_to.h
@@ -74,6 +74,8 @@ static inline void update_task_stack(str
 #endif
 }
 
+extern void kthread_from_fork(int (*fn)(void *), void *);
+
 static inline void kthread_frame_init(struct inactive_task_frame *frame,
 				      int (*fun)(void *), void *arg)
 {
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -544,6 +544,11 @@ void compat_start_thread(struct pt_regs
 }
 #endif
 
+__visible noinstr void kthread_from_fork(int (*fn)(void *), void *arg)
+{
+	fn(arg);
+}
+
 /*
  *	switch_to(x,y) should switch tasks from x to y.
  *