Messages in this thread | | | Date | Wed, 29 Jul 2015 17:23:29 -0400 | From | Boris Ostrovsky <> | Subject | Re: [Xen-devel] [PATCH v4 0/3] x86: modify_ldt improvement, test, and config option |
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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.
> > 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?
> > One solution might be to use MMU_NORMAL_PT_UPDATE hypercall instead, > which take the physical address of pte to update. This won't fail in > Xen if part of the paging structure is missing, and can be batched.
Yes, it does work. Thanks Andrew.
-boris
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