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Subject[patch 11/32] 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>
Cc: ebiederm@xmission.com (Eric W. Biederman)

---
arch/i386/xen/enlighten.c | 27 +++++++++++++++++++++++++--
1 file changed, 25 insertions(+), 2 deletions(-)
===================================================================
--- a/arch/i386/xen/enlighten.c
+++ b/arch/i386/xen/enlighten.c
@@ -635,7 +635,7 @@ static void xen_write_cr3(unsigned long

/* 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)));
@@ -687,9 +687,31 @@ static void *xen_kmap_atomic_pte(struct
}
#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;
/*
@@ -737,6 +759,7 @@ static __init void xen_pagetable_setup_d
/* 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)) {
/*
@@ -919,7 +942,7 @@ static const struct paravirt_ops xen_par
.dup_mmap = xen_dup_mmap,
.exit_mmap = xen_exit_mmap,

- .set_pte = xen_set_pte,
+ .set_pte = NULL, /* see xen_pagetable_setup_* */
.set_pte_at = xen_set_pte_at,
.set_pmd = xen_set_pmd,

--

-
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