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    SubjectRe: [PATCH RFC] x86: check for and defend against BIOS memory corruption
    On Fri, 29 Aug 2008, Jeremy Fitzhardinge wrote:
    > Hugh Dickins wrote:
    > > Is this the right moment for me to mention again that I'm not sure
    > > your reuse of existing pagetables was quite right anyway: NX being
    > > excluded from level2_ident_pgt, but wanted in the direct map?
    > We could add NX. What's the behaviour of setting NX in a
    > non-NX-supporting CPU? I don't think it would trigger a "reserved bit"
    > exception (the other high pte flags don't). Or failing that, we could
    > mask out NX once we've worked out the CPU doesn't support it (at the
    > same time it relocates the pagetables to the kernel's load-time address).

    I've no experience of what happens if NX is set to a non-NX-supporting
    CPU, so can't advise on that at all. But I think you're looking at
    it the wrong way round - or else I am.

    Here's the declaration and comment on level2_ident_pgt in head_64.S:

    /* Since I easily can, map the first 1G.
    * Don't set NX because code runs from these pages.

    (The "Since I easily can" comment is there because it used to map
    only a subset needed for early kernel startup, not the whole 1G.)

    So it's very intentionally leaving NX out there. I believe the
    level2_ident_pgt page appears twice or more in the pagetable layout,
    used to map two or more areas of virtual address space - once to
    provide the direct map at ffff880000000000 and once to provide
    the kernel image virtual mapping at ffffffff80200000.

    I think we don't want NX on Linux kernel text ;-?

    Before your 2.6.27-rc changes, init_memory_mapping subsequently
    replaced the direct map usage by a separately constructed pagetable,
    similar to it but with NX set throughout. After your 2.6.27-rc
    changes, level2_ident_pgt is found there already so left untouched -
    leaving NX out of that first 1G of the direct map forever (when CPA
    splits it up, the smaller pages inherit the lack of NX too).

    I noticed when changing /proc/meminfo to show DirectMap in kB,
    and tried to fix it, but only gave myself a non-booting system
    (I probably shrank my direct map to 0 while implicitly using it).
    And at that stage I didn't realize at all that it was a recent
    regression arising from your mods.

    I'm reluctant to delve in there again at present, and unsure
    what the right fix should be: perhaps the code which checks if
    an entry is already there, should check if it has the desired


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