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    SubjectRe: i686 quirk for AMD Geode

    > > >>> *THIS* is the kind of complexity that makes me think that having a
    > > >>> single source for all interpretation done in the kernel is the
    > > >>> preferred option.
    > > >>
    > > >> Definitely agreed ... The NX code is quite a maze right now, so changes
    > > >> to it should come generously laced with cleanups.
    > > >
    > > > BTW, I don't see why we should be impacted by NX. Trying to
    > > > execute from an NX page would return a SEGV, not SIGILL, so
    > > > we should not be bothered, am I wrong ?
    > >
    > > Yes. Consider a page-crossing instruction.
    > OK, but to be pragmatic, NX is there to prevent execution of
    > instructions in the stack (or heap) during buffer overflows.
    > If we only implement the few instructions lised in previous
    > mail, there is very little benefit to check for NX :
    > - those instructions cannot jump to other code, they just
    > change one register or memory location at most (or just nop)
    > - once we return from the signal handler, if we have crossed
    > a NX page boundary, the program will segfault anyway, taking
    > with it the change we just completed.
    > - last, the probability of having an NX page just after an
    > executable one seems too tight to me to even constitute
    > an attack vector ! BTW, I'm not even certain that all CPUs
    > correctly implement this check !

    Yes, you can probably "get away" with it. But I would not want to
    debug problems on systems with half-instruction-emulation. Please do
    it right, or not at all.

    > So in short, I think we could reasonably implement CMOV/NOPL
    > with the instruction length control, with getuser for data
    > accesses but without checking the code pages permissions if
    > we know that the CPU could already fetch the beginning of
    > the instruction correctly to cause an invalid opcode trap.
    > I'm not saying this is perfect, just that this is reasonable.

    Reasonable hack to get distro booting yes. Reasonable for mainline?

    (cesky, pictures)

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