On Wed, Nov 29, 2017 at 6:56 AM, David Laight <[email protected]> wrote: > 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.
SWAPGS never faults. I did test my patch, and I'm wondering how I missed the problem. Looking now...
