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    SubjectRe: [PATCH 4/7] x86: Add support for rd/wr fs/gs base
    On Thu, May 1, 2014 at 2:15 PM, Andi Kleen <andi@firstfloor.org> wrote:
    >> If usergs == kernelgs, then ebx will always be 1 and we'll never end
    >> up in paranoid_userspace.
    >
    > You may miss a reschedule in this obscure case. It shouldn't really
    > happen because loading a kernel pointer is not useful for user space.
    >
    > Doesn't seem like a real issue to me.
    >
    > We only happen need to handle it to avoid crashing.

    Allowing userspace to prevent itself from being rescheduled by loading
    something strange into gsbase seems unfortunate.

    --Andy

    >
    >> Alternatively, what if the paranoid entry checked whether we're coming
    >> from userspace at the very beginning and, if so, just jumped to the
    >> non-paranoid entry?
    >
    > That would work, but I doubt it would be worth it.
    >


    > -Andi



    --
    Andy Lutomirski
    AMA Capital Management, LLC


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