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Date:   Wed, 29 Nov 2017 14:56:43 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Andy Lutomirski' <luto@...capital.net>,
        Peter Zijlstra <peterz@...radead.org>
CC:     Ingo Molnar <mingo@...nel.org>,
        Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Borislav Petkov <bp@...en8.de>
Subject: RE: [PATCH] x86/entry/64: Fix native_load_gs_index() SWAPGS handling
 with IRQ state tracing enabled

From: Andy Lutomirski
> Sent: 29 November 2017 14:34
> > On Nov 29, 2017, at 4:47 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> >> On Wed, Nov 29, 2017 at 08:09:51AM +0100, Ingo Molnar wrote:
> >> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> >> index f81d50d7ceac..c0b52df8ee4f 100644
> >> --- a/arch/x86/entry/entry_64.S
> >> +++ b/arch/x86/entry/entry_64.S
> >> @@ -945,16 +945,16 @@ idtentry simd_coprocessor_error        do_simd_coprocessor_error
> has_error_code=0
> >>     */
> >> ENTRY(native_load_gs_index)
> >>    FRAME_BEGIN
> >> +    SWAPGS                    /* switch from user GS to kernel GS */
> >>    pushfq
> >>    DISABLE_INTERRUPTS(CLBR_ANY & ~CLBR_RDI)
> >>    TRACE_IRQS_OFF
> >> -    SWAPGS
> >
> > I'm thinking those moves it too far back; we should at least have
> > interrupts disabled when we do SWAPGS, no? Also, curse paravirt.
> >
> 
> I'll look in a few hours.
> 
> But we definitely can't have irqs on when running with user gs.

I can't remember what happens when swapgs itself faults.
I'm pretty sure it can, restoring %ds %es and %fs can definitely fault
(especially for 32 bit apps) if the restored values are invalid.
With user LDT I think it is possible for a valid segment register
to become invalid while a process is sleeping.
The debugger (and possible a signal handler) can set the segment
registers to arbitrary values - so loading them faults.

I fixed NetBSD a few years ago so that all these faults were handled
correctly.

	David


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