2017-11-29 18:19:43

by Andy Lutomirski

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 <[email protected]> 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.
From 1585425215511738930@xxx Wed Nov 29 18:15:08 +0000 2017
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2017-11-29 14:57:27

by David Laight

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 <[email protected]> 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



From 1585404210277459895@xxx Wed Nov 29 12:41:16 +0000 2017
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