linux-kernel - Re: CONFIG_PAGE_TABLE_ISOLATION=y on x86_64 causes gcc to segfault when building x86

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Date:   Wed, 3 Jan 2018 14:12:00 -0800
From:   Laura Abbott <labbott@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Lars Wendler <wendler.lars@....de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Borislav Betkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>
Subject: Re: CONFIG_PAGE_TABLE_ISOLATION=y on x86_64 causes gcc to segfault
 when building x86_32 binaries

On 01/03/2018 10:52 AM, Thomas Gleixner wrote:
> On Wed, 3 Jan 2018, Thomas Gleixner wrote:
> 
>> On Wed, 3 Jan 2018, Lars Wendler wrote:
>>> Am Wed, 3 Jan 2018 13:05:38 +0100 (CET)
>>> schrieb Thomas Gleixner <tglx@...utronix.de>:
>>>> Also can you please try Linus v4.15-rc6 with PTI enabled so we can see
>>>> whether that's a backport issue or a general one?
>>>
>>> Same problem with 4.15-rc6. So I suppose that means it's a general
>>> issue.
>>
>> Just a shot in the dark as I just decoded another issue on a AMD CPU. Can
>> you please try the patch below?
> 
> Ok. Found the real issue. This is a problem on AMD boxen.
> 

Fedora reporter says it fixes it.

> Fix below.
> 
> Can Xen folks please have a look at that as well?
> 
> Thanks,
> 
> 	tglx
> 
> 8<-------------------
> 
> arch/x86/entry/entry_64_compat.S |   13 ++++++-------
>   1 file changed, 6 insertions(+), 7 deletions(-)
> 
> --- a/arch/x86/entry/entry_64_compat.S
> +++ b/arch/x86/entry/entry_64_compat.S
> @@ -190,8 +190,13 @@ ENTRY(entry_SYSCALL_compat)
>   	/* Interrupts are off on entry. */
>   	swapgs
>   
> -	/* Stash user ESP and switch to the kernel stack. */
> +	/* Stash user ESP */
>   	movl	%esp, %r8d
> +
> +	/* Use %rsp as scratch reg. User ESP is stashed in r8 */
> +	SWITCH_TO_KERNEL_CR3 scratch_reg=%rsp
> +	
> +	/* Switch to the kernel stack */
>   	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rsp
>   
>   	/* Construct struct pt_regs on stack */
> @@ -220,12 +225,6 @@ GLOBAL(entry_SYSCALL_compat_after_hwfram
>   	pushq   $0			/* pt_regs->r15 = 0 */
>   
>   	/*
> -	 * We just saved %rdi so it is safe to clobber.  It is not
> -	 * preserved during the C calls inside TRACE_IRQS_OFF anyway.
> -	 */
> -	SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi
> -
> -	/*
>   	 * User mode is traced as though IRQs are on, and SYSENTER
>   	 * turned them off.
>   	 */
>