linux-kernel - Re: [PATCH] x86/entry/64: Fix native_load_gs_index() SWAPGS handling with IRQ state tracing enabled

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Date:   Wed, 29 Nov 2017 06:33:37 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     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



> 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.