Re: [PATCH 12/31] x86/entry/32: Add PTI cr3 switch to non-NMI entry/exit points

[Joerg Roedel]

On Tue, Feb 27, 2018 at 02:18:36PM -0500, Waiman Long wrote:
> > +	/* Make sure we are running on kernel cr3 */
> > +	SWITCH_TO_KERNEL_CR3 scratch_reg=%eax
> > +
> >  	xorl	%edx, %edx			# error code 0
> >  	movl	%esp, %eax			# pt_regs pointer
> >  
> 
> The debug exception calls ret_from_exception on exit. If coming from
> userspace, the C function prepare_exit_to_usermode() will be called.
> With the PTI-32 code, it means that function will be called with the
> entry stack instead of the task stack. This can be problematic as macro
> like current won't work anymore.

Okay, I had another look at the debug handler. As I said before, it
already handles the from-entry-stack case, but with these patches it
gets more likely that we actually hit that path.

Also, with the special handling for from-kernel-with-entry-stack
situations we can simplify the debug handler and make it more robust
with the diff below. Thoughts?

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 8c149f5..844aff1 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -1318,33 +1318,14 @@ ENTRY(debug)
 	ASM_CLAC
 	pushl	$-1				# mark this as an int
 
-	SAVE_ALL
+	SAVE_ALL switch_stacks=1
 	ENCODE_FRAME_POINTER
 
-	/* Make sure we are running on kernel cr3 */
-	SWITCH_TO_KERNEL_CR3 scratch_reg=%eax
-
 	xorl	%edx, %edx			# error code 0
 	movl	%esp, %eax			# pt_regs pointer
 
-	/* Are we currently on the SYSENTER stack? */
-	movl	PER_CPU_VAR(cpu_entry_area), %ecx
-	addl	$CPU_ENTRY_AREA_entry_stack + SIZEOF_entry_stack, %ecx
-	subl	%eax, %ecx	/* ecx = (end of entry_stack) - esp */
-	cmpl	$SIZEOF_entry_stack, %ecx
-	jb	.Ldebug_from_sysenter_stack
-
-	TRACE_IRQS_OFF
-	call	do_debug
-	jmp	ret_from_exception
-
-.Ldebug_from_sysenter_stack:
-	/* We're on the SYSENTER stack.  Switch off. */
-	movl	%esp, %ebx
-	movl	PER_CPU_VAR(cpu_current_top_of_stack), %esp
 	TRACE_IRQS_OFF
 	call	do_debug
-	movl	%ebx, %esp
 	jmp	ret_from_exception
 END(debug)