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 14:27:17 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Lars Wendler <wendler.lars@....de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Borislav Betkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Laura Abbott <labbott@...hat.com>,
        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 02:14 PM, Andy Lutomirski wrote:
> +       /* 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

The stack is unreadable at this point without the CR3 switch.

> What's the issue that this is fixing?

Users doing 32-bit SYSCALLs on the CPUs that support them double fault
since they end up with an %rsp that they can't access.