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Date:   Wed, 29 Nov 2017 08:51:45 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     David Laight <David.Laight@...lab.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.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

On Wed, Nov 29, 2017 at 8:22 AM, Andy Lutomirski <luto@...nel.org> wrote:
> On Wed, Nov 29, 2017 at 6:56 AM, David Laight <David.Laight@...lab.com> wrote:
>> 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.
>
> SWAPGS never faults.
>
> I did test my patch, and I'm wondering how I missed the problem.  Looking now...

I think my patch was entirely correct and Jarkko's problem is some
other bug.  Lemme see if I can find it.

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