Messages in this thread | | | From | Andy Lutomirski <> | Date | Wed, 29 Nov 2017 08:51:45 -0800 | Subject | Re: [PATCH] x86/entry/64: Fix native_load_gs_index() SWAPGS handling with IRQ state tracing enabled |
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On Wed, Nov 29, 2017 at 8:22 AM, Andy Lutomirski <luto@kernel.org> wrote: > On Wed, Nov 29, 2017 at 6:56 AM, David Laight <David.Laight@aculab.com> wrote: >> From: Andy Lutomirski >>> Sent: 29 November 2017 14:34 >>> > On Nov 29, 2017, at 4:47 AM, Peter Zijlstra <peterz@infradead.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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