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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