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From: Andy Lutomirski <luto@kernel.org>
Date: Wed, 3 Jan 2018 14:14:52 -0800
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Subject: Re: CONFIG_PAGE_TABLE_ISOLATION=y on x86_64 causes gcc to segfault
 when building x86_32 binaries
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Lars Wendler <wendler.lars@web.de>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Borislav Betkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        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>
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On Wed, Jan 3, 2018 at 10:52 AM, Thomas Gleixner <tglx@linutronix.de> 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@linutronix.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.
>
> 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.
>          */

What's the issue that this is fixing?