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    SubjectRE: [PATCH] x86/entry/64: Fix native_load_gs_index() SWAPGS handling with IRQ state tracing enabled
    Date
    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


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