Date: Wed, 3 Jan 2018 23:22:57 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Andy Lutomirski <luto@kernel.org>
cc: Lars Wendler <wendler.lars@web.de>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Borislav Betkov <bp@alien8.de>,
        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>
Subject: Re: CONFIG_PAGE_TABLE_ISOLATION=y on x86_64 causes gcc to segfault
 when building x86_32 binaries
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On Wed, 3 Jan 2018, Andy Lutomirski wrote:

> 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?

> >         movq    PER_CPU_VAR(cpu_current_top_of_stack), %rsp

before switching CR3 is obviously broken ...