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    SubjectRe: Avoid speculative indirect calls in kernel
    On Tue, 9 Jan 2018, Dave Hansen wrote:
    > On 01/09/2018 04:45 PM, Thomas Gleixner wrote:
    > > On Mon, 8 Jan 2018, Andrea Arcangeli wrote:
    > >> On Mon, Jan 08, 2018 at 09:53:02PM +0100, Thomas Gleixner wrote:
    > >> Did my best to do the cleanest patch for tip, but I now figured Dave's
    > >> original comment was spot on: a _PAGE_NX clear then becomes necessary
    > >> also after pud_alloc not only after p4d_alloc.
    > >>
    > >> pmd_alloc would run into the same with x86 32bit non-PAE too.
    >
    > non-PAE doesn't have an NX bit. :)
    >
    > But we #define _PAGE_NX down to 0 there so it's harmless.
    >
    > >> So there are two choices, either going back to one single _PAGE_NX
    > >> clear from the original Dave's original patch as below, or to add
    > >> multiple clear after each level which was my objective and is more
    > >> robust, but it may be overkill in this case. As long as it was one
    > >> line it looked a clear improvement.
    > >>
    > >> Considering the caller in both cases is going to abort I guess we can
    > >> use the one liner approach as Dave and Jiri did originally.
    > >
    > > Dave ?
    >
    > I agree with Andrea. The patch in -tip potentially misses the pgd
    > clearing if pud_alloc() sets a PGD. It would also be nice to have that
    > comment back.
    >
    > Note that the -tip commit probably works in *practice* because for two
    > adjacent calls to map_tboot_page() that share a PGD entry, the first
    > will clear NX, *then* allocate and set the PGD (without NX clear). The
    > second call will *not* allocate but will clear the NX bit.
    >
    > The patch I think we want is attached.

    Color me confused. I have queued the one below in tip. It lacks the comment
    and does the !NX at a different place.

    Thanks,

    tglx

    8<-----------------

    commit 262b6b30087246abf09d6275eb0c0dc421bcbe38
    Author: Dave Hansen <dave.hansen@linux.intel.com>
    Date: Sat Jan 6 18:41:14 2018 +0100

    x86/tboot: Unbreak tboot with PTI enabled

    This is another case similar to what EFI does: create a new set of
    page tables, map some code at a low address, and jump to it. PTI
    mistakes this low address for userspace and mistakenly marks it
    non-executable in an effort to make it unusable for userspace.

    Undo the poison to allow execution.

    Fixes: 385ce0ea4c07 ("x86/mm/pti: Add Kconfig")
    Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
    Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Cc: Alan Cox <gnomes@lxorguk.ukuu.org.uk>
    Cc: Tim Chen <tim.c.chen@linux.intel.com>
    Cc: Jon Masters <jcm@redhat.com>
    Cc: Dave Hansen <dave.hansen@intel.com>
    Cc: Andi Kleen <andi@firstfloor.org>
    Cc: Jeff Law <law@redhat.com>
    Cc: Paolo Bonzini <pbonzini@redhat.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Greg Kroah-Hartman <gregkh@linux-foundation.org>
    Cc: David" <dwmw@amazon.co.uk>
    Cc: Nick Clifton <nickc@redhat.com>
    Cc: stable@vger.kernel.org
    Link: https://lkml.kernel.org/r/20180108102805.GK25546@redhat.com

    diff --git a/arch/x86/kernel/tboot.c b/arch/x86/kernel/tboot.c
    index a4eb27918ceb..75869a4b6c41 100644
    --- a/arch/x86/kernel/tboot.c
    +++ b/arch/x86/kernel/tboot.c
    @@ -127,6 +127,7 @@ static int map_tboot_page(unsigned long vaddr, unsigned long pfn,
    p4d = p4d_alloc(&tboot_mm, pgd, vaddr);
    if (!p4d)
    return -1;
    + pgd->pgd &= ~_PAGE_NX;
    pud = pud_alloc(&tboot_mm, p4d, vaddr);
    if (!pud)
    return -1;
    \
     
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