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

[Andrew Cooper]

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.

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.

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.

~Andrew
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