Re: [Xen-devel] [PATCH v4 0/3] x86: modify_ldt improvement, test, and config option

[Boris Ostrovsky]

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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