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SubjectRe: CONFIG_PAGE_TABLE_ISOLATION=y on x86_64 causes gcc to segfault when building x86_32 binaries
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 ...



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