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    SubjectRe: [Xen-devel] [PATCH v4 0/3] x86: modify_ldt improvement, test, and config option
    On 07/30/2015 02:54 PM, Andrew Cooper wrote:
    > On 30/07/15 19:30, Andy Lutomirski wrote:
    >> On Wed, Jul 29, 2015 at 5:29 PM, Andrew Cooper
    >> <andrew.cooper3@citrix.com> wrote:
    >>> On 30/07/2015 00:13, Andy Lutomirski wrote:
    >>>> On Wed, Jul 29, 2015 at 4:02 PM, Andrew Cooper
    >>>> <andrew.cooper3@citrix.com> wrote:
    >>>>> On 29/07/2015 23:49, Boris Ostrovsky wrote:
    >>>>>> On 07/29/2015 06:46 PM, David Vrabel wrote:
    >>>>>>> On 29/07/2015 23:11, Andrew Cooper wrote:
    >>>>>>>> On 29/07/2015 23:05, Andy Lutomirski wrote:
    >>>>>>>>> On Wed, Jul 29, 2015 at 2:37 PM, Andrew Cooper
    >>>>>>>>> <andrew.cooper3@citrix.com> wrote:
    >>>>>>>>>> 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.
    >>>>>>>>> __get_user((char *)v, tmp), perhaps, unless there's something better
    >>>>>>>>> in the wings. Keep in mind that we need this for -stable, and it's
    >>>>>>>>> likely to get backported quite quickly due to CVE-2015-5157.
    >>>>>>>> Hmm - something like that tucked inside HYPERVISOR_update_va_mapping()
    >>>>>>>> would probably work, and certainly be minimal hassle for -stable.
    >>>>>>>>
    >>>>>>>> Altering the hypercall used is certainly not something to backport, nor
    >>>>>>>> are we sure it is a viable fix at this time.
    >>>>>>> Changing this one use of update_va_mapping to use mmu_update_normal_pt
    >>>>>>> is the correct fix to unblock this LDT series. I see no reason why this
    >>>>>>> cannot be backported.
    >>>>>> To properly fix it should include batching and that is not something
    >>>>>> that I think we should target for stable.
    >>>>> Batching is absolutely not necessary to alter update_va_mapping to
    >>>>> mmu_update_normal_pt. After all, update_va_mapping isn't batched.
    >>>>>
    >>>>> However this isn't the first issue issue we have had lazy mmu faulting,
    >>>>> and I doubt it is the last. There are not many callsites of
    >>>>> update_va_mapping - I will audit them tomorrow and see if any similar
    >>>>> issues are lurking elsewhere.
    >>>> One thing I should add: nothing flushes old aliases in xen_alloc_ldt,
    >>>> yet I haven't been able to get xen_alloc_ldt to fail or subsequent LDT
    >>>> access to fault. Is this something we should be worried about?
    >>> Yes. update_va_mapping() will function perfectly well taking one RW
    >>> mapping to RO even if there is a second RW mapping. In such a case, the
    >>> next LDT access will fault.
    >> Which is a problem because that alias might still exist, and also
    >> because Linux really doesn't expect that fault.
    >>
    >>> On closer inspection, Xen is rather unhelpful with the fault. Xen's
    >>> lazy #PF will be bounced back to the guest with cr2 adjusted to appear
    >>> in the range passed to set_ldt(). The error code however will be
    >>> unmodified (and limited only by not-user and not-reserved), so will
    >>> appear as a non-present read or write supervisor access to an address
    >>> which the kernel has a valid read mapping of.
    >> More yuck.
    >>
    >> I think I'm just going to stick an unconditional vm_flush_aliases in alloc_ldt.
    >>
    >>> Therefore, set_ldt() needs to be confident that there are no writeable
    >>> mappings to the frames used to make up the LDT. It could proactively
    >>> fault them in by accessing one descriptor in each page inside the limit,
    >>> but by the time a fault is received it is probably too late to work out
    >>> where the other mapping is which prevented the typechange (or indeed,
    >>> whether Xen objected to one of the descriptors instead).
    >> This seems like overkill.
    >>
    >> I'm still a bit confused, though: the failure is in xen_free_ldt. How
    >> do we make it all the way to xen_free_ldt without the vmapped page
    >> existing in the guest's page tables? After all, we had to survive
    >> xen_alloc_ldt first, and ISTM that should fail in exactly the same
    >> way.
    > (Summarising part of a discussion which has just occurred on IRC)
    >
    > I presume that xen_free_ldt() is called while in the context of an mm
    > which doesn't have the particular area of the vmalloc() space faulted in.

    This is exactly what's happening --- the bug is only triggered during
    exit and xen_free_ldt() is called from someone else's context, e.g.:

    [ 53.986677] Call Trace:
    [ 53.986677] [<c105312d>] xen_free_ldt+0x2d/0x40
    [ 53.986677] [<c1062310>] free_ldt_struct.part.1+0x10/0x40
    [ 53.986677] [<c1062735>] destroy_context+0x25/0x40
    [ 53.986677] [<c10a764e>] __mmdrop+0x1e/0xc0
    [ 53.986677] [<c10c9858>] finish_task_switch+0xd8/0x1a0
    [ 53.986677] [<c1863736>] __schedule+0x316/0x950
    [ 53.986677] [<c1863d96>] schedule+0x26/0x70
    [ 53.986677] [<c10ac613>] do_wait+0x1b3/0x200
    [ 53.986677] [<c10ac9d7>] SyS_waitpid+0x67/0xd0
    [ 53.986677] [<c10aa820>] ? task_stopped_code+0x50/0x50
    [ 53.986677] [<c186717a>] syscall_call+0x7/0x7

    But that would imply that this other context has mm->context.ldt of
    ldt_gdt_32. How is that possible?

    -boris

    >
    > This is (I presume) why reading 'v' (which occasionally causes a
    > pagefault to occur) fixes the issue.
    >
    > ~Andrew



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