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Subject: Re: CONFIG_PAGE_TABLE_ISOLATION=y on x86_64 causes gcc to segfault
 when building x86_32 binaries
To: Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
References: <20180103123723.1dd26828@abudhabi.paradoxon.rec>
 <alpine.DEB.2.20.1801031256440.1957@nanos>
 <alpine.DEB.2.20.1801031259530.1957@nanos>
 <20180103143036.60e592eb@abudhabi.paradoxon.rec>
 <alpine.DEB.2.20.1801031620020.1957@nanos>
 <alpine.DEB.2.20.1801031949200.1957@nanos>
 <CALCETrW8RD2vtcUjYoq07DEiLoXYxqXuxD7xujOtuF=UGjNYvw@mail.gmail.com>
Cc: Lars Wendler <wendler.lars@web.de>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Borislav Betkov <bp@alien8.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Laura Abbott <labbott@redhat.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>
From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <e29cbd54-95f2-52ed-508e-12c1c2520bfb@linux.intel.com>
Date: Wed, 3 Jan 2018 14:27:17 -0800
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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.