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)

1 files changed, 25 insertions, 2 deletions

diff --git a/arch/i386/xen/enlighten.c b/arch/i386/xen/enlighten.c
index 86e68e680116..9550ae3b1fb1 100644
--- a/arch/i386/xen/enlighten.c
+++ b/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,