Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH] x86/entry/64: Fix native_load_gs_index() SWAPGS handling with IRQ state tracing enabled | Date | Wed, 29 Nov 2017 14:56:43 +0000 |
| |
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.
David
| |