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    SubjectRe: [RFC PATCH 5/7] x86/mm/fault: hook up SCI verification
    On Sat, Apr 27, 2019 at 10:47 PM Mike Rapoport <rppt@linux.ibm.com> wrote:
    >
    > On Fri, Apr 26, 2019 at 09:42:23AM +0200, Peter Zijlstra wrote:
    > > On Fri, Apr 26, 2019 at 12:45:52AM +0300, Mike Rapoport wrote:
    > > > If a system call runs in isolated context, it's accesses to kernel code and
    > > > data will be verified by SCI susbsytem.
    > > >
    > > > Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
    > > > ---
    > > > arch/x86/mm/fault.c | 28 ++++++++++++++++++++++++++++
    > > > 1 file changed, 28 insertions(+)
    > >
    > > There's a distinct lack of touching do_double_fault(). It appears to me
    > > that you'll instantly trigger #DF when you #PF, because the #PF handler
    > > itself will not be able to run.
    >
    > The #PF handler is able to run. On interrupt/error entry the cr3 is
    > switched to the full kernel page tables, pretty much like PTI does for
    > user <-> kernel transitions. It's in the patch 3.
    >
    >

    PeterZ meant page_fault, not do_page_fault. In your patch, page_fault
    and some of error_entry run before that magic switchover happens. If
    they're not in the page tables, you double-fault.

    And don't even try to do SCI magic in the double-fault handler. As I
    understand it, the SDM and APM aren't kidding when they say that #DF
    is an abort, not a fault. There is a single case in the kernel where
    we recover from #DF, and it was vetted by microcode people.

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