i386: fix lazy mode vmalloc synchronization for paravirt

Found this looping Ubuntu installs with VMI.

If unlucky enough to hit a vmalloc sync fault during a lazy mode
operation (from an IRQ handler for a module which was not yet populated
in current page directory, or from inside copy_one_pte, which touches
swap_map, and hit in an unused 4M region), the required PDE update would
never get flushed, causing an infinite page fault loop.

This bug affects any paravirt-ops backend which uses lazy updates, I
believe that makes it a bug in Xen, VMI and lguest.  It only happens on
LOWMEM kernels.


Touching vmalloc memory in the middle of a lazy mode update can generate a
kernel PDE update, which must be flushed immediately.  The fix is to leave
lazy mode when doing a vmalloc sync.

Signed-off-by: Zachary Amsden <zach@vmware.com>
Cc: Andi Kleen <ak@suse.de>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

arch/i386/mm/fault.c

@@ -249,9 +249,10 @@ static inline pmd_t *vmalloc_sync_one(pgd_t *pgd, unsigned long address)
 	pmd_k = pmd_offset(pud_k, address);
 	if (!pmd_present(*pmd_k))
 		return NULL;
-	if (!pmd_present(*pmd))
+	if (!pmd_present(*pmd)) {
 		set_pmd(pmd, *pmd_k);
-	else
+		arch_flush_lazy_mmu_mode();
+	} else
 		BUG_ON(pmd_page(*pmd) != pmd_page(*pmd_k));
 	return pmd_k;
 }