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    SubjectRe: [Xen-devel] [PATCH v4 0/3] x86: modify_ldt improvement, test, and config option
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    On 29/07/2015 22:26, Andy Lutomirski wrote:
    > On Wed, Jul 29, 2015 at 2:23 PM, Boris Ostrovsky
    > <boris.ostrovsky@oracle.com> wrote:
    >> On 07/29/2015 03:03 PM, Andrew Cooper wrote:
    >>> On 29/07/15 15:43, Boris Ostrovsky wrote:
    >>>> FYI, I have got a repro now and am investigating.
    >>> Good and bad news. This bug has nothing to do with LDTs themselves.
    >>>
    >>> I have worked out what is going on, but this:
    >>>
    >>> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
    >>> index 5abeaac..7e1a82e 100644
    >>> --- a/arch/x86/xen/enlighten.c
    >>> +++ b/arch/x86/xen/enlighten.c
    >>> @@ -493,6 +493,7 @@ static void set_aliased_prot(void *v, pgprot_t prot)
    >>> pte = pfn_pte(pfn, prot);
    >>> + (void)*(volatile int*)v;
    >>> if (HYPERVISOR_update_va_mapping((unsigned long)v, pte, 0)) {
    >>> pr_err("set_aliased_prot va update failed w/ lazy mode
    >>> %u\n", paravirt_get_lazy_mode());
    >>> BUG();
    >>>
    >>> Is perhaps not the fix we are looking for, and every use of
    >>> HYPERVISOR_update_va_mapping() is susceptible to the same problem.
    >>
    >> I think in most cases we know that page is mapped so hopefully this is the
    >> only site that we need to be careful about.
    > Is there any chance we can get some kind of quick-and-dirty fix that
    > can go to x86/urgent in the next few days even if a clean fix isn't
    > available yet?

    Quick and dirty?

    Reading from v is the most obvious and quick way, for areas where we are
    certain v exists, is kernel memory and is expected to have a backing
    page. I don't know offhand how many of current
    HYPERVISOR_update_va_mapping() callsites this applies to.

    >
    >>> The update_va_mapping hypercall is designed to emulate writing the pte
    >>> for v, with auditing applied. As part of this, it does a pagewalk on v
    >>> to locate and map the l1. During this walk, Xen it finds the l2 not
    >>> present, and fails the hypercall. i.e. v is not reachable from the
    >>> current cr3.
    >>>
    >>> Reading the virtual address immediately before issuing the hypercall
    >>> causes Linux's memory faulting logic to fault in the l2. This also
    >>> explains why vm_unmap_aliases() appears to fix the issue; it is likely
    >>> to fault in enough of the paging structure for v to be reachable.
    >>
    >> We've just touched this page (in write_ldt()) in this test so why would it
    >> not be mapped?
    > With my patches applied, the LDT is never written via any paravirt
    > hook -- I write it once (possibly implicitly using kzalloc/vzalloc)
    > before paravirt_alloc_ldt(), and write_ldt() is never called. We
    > could even remove it write_ldt() :)

    Even better!

    ~Andrew


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