linux-kernel - Re: CONFIG_PAGE_TABLE_ISOLATION=y on x86_64 causes gcc to segfault when building x86

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Date:   Wed, 3 Jan 2018 19:52:04 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Lars Wendler <wendler.lars@....de>
cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Borislav Betkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Laura Abbott <labbott@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>
Subject: Re: CONFIG_PAGE_TABLE_ISOLATION=y on x86_64 causes gcc to segfault
 when building x86_32 binaries

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@...utronix.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.
 	 */