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

[Laura Abbott]

On 01/03/2018 10:52 AM, Thomas Gleixner 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@xxxxxxxxxxxxx>:
> > > > 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.

Fedora reporter says it fixes it.

> 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.
>   	 */