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    SubjectRe: [Xen-devel] [PATCH v4 0/3] x86: modify_ldt improvement, test, and config option
    On Thu, Jul 30, 2015 at 1:01 PM, Boris Ostrovsky
    <boris.ostrovsky@oracle.com> wrote:
    > 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?
    >

    It's freed via destroy_context, which destroys someone else's LDT, right?

    --Andy


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