xen: ignore RW mapping of RO pages in pagetable_init

When setting up the initial pagetable, which includes mappings of all
low physical memory, ignore a mapping which tries to set the RW bit on
an RO pte.  An RO pte indicates a page which is part of the current
pagetable, and so it cannot be allowed to become RW.

Once xen_pagetable_setup_done is called, set_pte reverts to its normal
behaviour.
Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Acked-by: Chris Wright <chrisw@sous-sol.org>
Cc: ebiederm@xmission.com (Eric W. Biederman)

arch/i386/xen/enlighten.c

@@ -505,7 +505,7 @@ static void xen_write_cr3(unsigned long cr3)
 
 /* Early in boot, while setting up the initial pagetable, assume
    everything is pinned. */
-static void xen_alloc_pt_init(struct mm_struct *mm, u32 pfn)
+static __init void xen_alloc_pt_init(struct mm_struct *mm, u32 pfn)
 {
 	BUG_ON(mem_map);	/* should only be used early */
 	make_lowmem_page_readonly(__va(PFN_PHYS(pfn)));
@@ -557,10 +557,32 @@ static void *xen_kmap_atomic_pte(struct page *page, enum km_type type)
 }
 #endif
 
+static __init pte_t mask_rw_pte(pte_t *ptep, pte_t pte)
+{
+	/* If there's an existing pte, then don't allow _PAGE_RW to be set */
+	if (pte_val_ma(*ptep) & _PAGE_PRESENT)
+		pte = __pte_ma(((pte_val_ma(*ptep) & _PAGE_RW) | ~_PAGE_RW) &
+			       pte_val_ma(pte));
+
+	return pte;
+}
+
+/* Init-time set_pte while constructing initial pagetables, which
+   doesn't allow RO pagetable pages to be remapped RW */
+static __init void xen_set_pte_init(pte_t *ptep, pte_t pte)
+{
+	pte = mask_rw_pte(ptep, pte);
+
+	xen_set_pte(ptep, pte);
+}
+
 static __init void xen_pagetable_setup_start(pgd_t *base)
 {
 	pgd_t *xen_pgd = (pgd_t *)xen_start_info->pt_base;
 
+	/* special set_pte for pagetable initialization */
+	paravirt_ops.set_pte = xen_set_pte_init;
+
 	init_mm.pgd = base;
 	/*
 	 * copy top-level of Xen-supplied pagetable into place.	 For
@@ -607,6 +629,7 @@ static __init void xen_pagetable_setup_done(pgd_t *base)
 	/* This will work as long as patching hasn't happened yet
 	   (which it hasn't) */
 	paravirt_ops.alloc_pt = xen_alloc_pt;
+	paravirt_ops.set_pte = xen_set_pte;
 
 	if (!xen_feature(XENFEAT_auto_translated_physmap)) {
 		/*
@@ -745,7 +768,7 @@ static const struct paravirt_ops xen_paravirt_ops __initdata = {
 	.kmap_atomic_pte = xen_kmap_atomic_pte,
 #endif
 
-	.set_pte = xen_set_pte,
+	.set_pte = NULL,	/* see xen_pagetable_setup_* */
 	.set_pte_at = xen_set_pte_at,
 	.set_pmd = xen_set_pmd,