linux-kernel - Re: [PATCH] x86/entry/64: Fix CR3 restore order in paranoid

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Date:   Wed, 14 Feb 2018 08:39:11 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] x86/entry/64: Fix CR3 restore order in paranoid_exit()


* Dave Hansen <dave.hansen@...el.com> wrote:

> On 02/13/2018 06:27 PM, Josh Poimboeuf wrote:
> > --- a/arch/x86/entry/entry_64.S
> > +++ b/arch/x86/entry/entry_64.S
> > @@ -1167,10 +1167,10 @@ ENTRY(paranoid_exit)
> >  	UNWIND_HINT_REGS
> >  	DISABLE_INTERRUPTS(CLBR_ANY)
> >  	TRACE_IRQS_OFF_DEBUG
> > +	RESTORE_CR3	scratch_reg=%r15 save_reg=%r14
> >  	testl	%ebx, %ebx			/* swapgs needed? */
> >  	jnz	.Lparanoid_exit_no_swapgs
> >  	TRACE_IRQS_IRETQ
> > -	RESTORE_CR3	scratch_reg=%rbx save_reg=%r14
> >  	SWAPGS_UNSAFE_STACK
> >  	jmp	.Lparanoid_exit_restore
> >  .Lparanoid_exit_no_swapgs:
> 
> TRACE_IRQS_* call non-entry functions that are not mapped by the user
> CR3.  How can this possibly work?  What am I missing?

How about something like the patch below? (Totally untested)

Thanks,

	Ingo
---
 arch/x86/entry/entry_64.S | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index cd216c9431e1..8971bd64d515 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1175,6 +1175,7 @@ ENTRY(paranoid_exit)
 	jmp	.Lparanoid_exit_restore
 .Lparanoid_exit_no_swapgs:
 	TRACE_IRQS_IRETQ_DEBUG
+	RESTORE_CR3	scratch_reg=%rbx save_reg=%r14
 .Lparanoid_exit_restore:
 	jmp restore_regs_and_return_to_kernel
 END(paranoid_exit)