x86/entry/32: Pass cr2 to do_async_page_fault()

Commit a0d14b89 ("x86/mm, tracing: Fix CR2 corruption") added the
address parameter to do_async_page_fault(), but does not pass it from the
32-bit entry point.  To plumb it through, factor-out
common_exception_read_cr2 in the same fashion as common_exception, and uses
it from both page_fault and async_page_fault.

For a 32-bit KVM guest, this fixes:

  Run /sbin/init as init process
  Starting init: /sbin/init exists but couldn't execute it (error -14)

Fixes: a0d14b89 ("x86/mm, tracing: Fix CR2 corruption")
Signed-off-by: Matt Mullins <mmullins@fb.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20190724042058.24506-1-mmullins@fb.com

arch/x86/entry/entry_32.S

@@ -1443,8 +1443,12 @@ BUILD_INTERRUPT3(hv_stimer0_callback_vector, HYPERV_STIMER0_VECTOR,
 
 ENTRY(page_fault)
 	ASM_CLAC
-	pushl	$0; /* %gs's slot on the stack */
+	pushl	$do_page_fault
+	jmp	common_exception_read_cr2
+END(page_fault)
 
+common_exception_read_cr2:
+	/* the function address is in %gs's slot on the stack */
 	SAVE_ALL switch_stacks=1 skip_gs=1
 
 	ENCODE_FRAME_POINTER
@@ -1452,6 +1456,7 @@ ENTRY(page_fault)
 
 	/* fixup %gs */
 	GS_TO_REG %ecx
+	movl	PT_GS(%esp), %edi
 	REG_TO_PTGS %ecx
 	SET_KERNEL_GS %ecx
 
@@ -1463,9 +1468,9 @@ ENTRY(page_fault)
 
 	TRACE_IRQS_OFF
 	movl	%esp, %eax			# pt_regs pointer
-	call	do_page_fault
+	CALL_NOSPEC %edi
 	jmp	ret_from_exception
-END(page_fault)
+END(common_exception_read_cr2)
 
 common_exception:
 	/* the function address is in %gs's slot on the stack */
@@ -1595,7 +1600,7 @@ END(general_protection)
 ENTRY(async_page_fault)
 	ASM_CLAC
 	pushl	$do_async_page_fault
-	jmp	common_exception
+	jmp	common_exception_read_cr2
 END(async_page_fault)
 #endif