xen: correct race in alloc_p2m_pmd()

When allocating a new pmd for the linear mapped p2m list a check is
done for not introducing another pmd when this just happened on
another cpu. In this case the old pte pointer was returned which
points to the p2m_missing or p2m_identity page. The correct value
would be the pointer to the found new page.
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

arch/x86/xen/p2m.c

@@ -439,10 +439,9 @@ EXPORT_SYMBOL_GPL(get_phys_to_machine);
  * a new pmd is to replace p2m_missing_pte or p2m_identity_pte by a individual
  * pmd. In case of PAE/x86-32 there are multiple pmds to allocate!
  */
-static pte_t *alloc_p2m_pmd(unsigned long addr, pte_t *ptep, pte_t *pte_pg)
+static pte_t *alloc_p2m_pmd(unsigned long addr, pte_t *pte_pg)
 {
 	pte_t *ptechk;
-	pte_t *pteret = ptep;
 	pte_t *pte_newpg[PMDS_PER_MID_PAGE];
 	pmd_t *pmdp;
 	unsigned int level;
@@ -476,8 +475,6 @@ static pte_t *alloc_p2m_pmd(unsigned long addr, pte_t *ptep, pte_t *pte_pg)
 		if (ptechk == pte_pg) {
 			set_pmd(pmdp,
 				__pmd(__pa(pte_newpg[i]) | _KERNPG_TABLE));
-			if (vaddr == (addr & ~(PMD_SIZE - 1)))
-				pteret = pte_offset_kernel(pmdp, addr);
 			pte_newpg[i] = NULL;
 		}
 
@@ -491,7 +488,7 @@ static pte_t *alloc_p2m_pmd(unsigned long addr, pte_t *ptep, pte_t *pte_pg)
 		vaddr += PMD_SIZE;
 	}
 
-	return pteret;
+	return lookup_address(addr, &level);
 }
 
 /*
@@ -520,7 +517,7 @@ static bool alloc_p2m(unsigned long pfn)
 
 	if (pte_pg == p2m_missing_pte || pte_pg == p2m_identity_pte) {
 		/* PMD level is missing, allocate a new one */
-		ptep = alloc_p2m_pmd(addr, ptep, pte_pg);
+		ptep = alloc_p2m_pmd(addr, pte_pg);
 		if (!ptep)
 			return false;
 	}