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

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Date:   Thu, 1 Mar 2018 14:34:30 +0100
From:   Joerg Roedel <joro@...tes.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>, aliguori@...zon.com,
        daniel.gruss@...k.tugraz.at, hughd@...gle.com, keescook@...gle.com,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, Pavel Machek <pavel@....cz>,
        jroedel@...e.de
Subject: Re: [PATCH 12/31] x86/entry/32: Add PTI cr3 switch to non-NMI
 entry/exit points

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)