x86: add locking to map_pages_to_xen()

While boot time calls don't need this, run time uses of the function
which may result in L2 page tables getting populated need to be
serialized to avoid two CPUs populating the same L2 (or L3) entry,
overwriting each other's results.

This is expected to fix what would seem to be a regression from commit
b0581b92 ("x86: make map_domain_page_global() a simple wrapper around
vmap()"), albeit that change only made more readily visible the already
existing issue.

This patch intentionally does not
- add locking to the page table de-allocation logic in
  destroy_xen_mappings() (the only user having potential races here,
  msix_put_fixmap(), gets converted to use __set_fixmap() instead)
- avoid races between super page splitting and reconstruction in
  map_pages_to_xen() (no such uses exist; races between multiple
  splitting attempts or between multiple reconstruction attempts are
  being taken care of)
If we wanted to take care of these, we'd need to alter the behavior
of virt_to_xen_l?e() - they would need to return with the lock held
then.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>

1 files changed, 167 insertions, 14 deletions

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index f151dec2b2..286e903182 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -5320,17 +5320,111 @@ void free_xen_pagetable(void *v)
         free_xenheap_page(v);
 }
 
+static DEFINE_SPINLOCK(map_pgdir_lock);
+
+static l3_pgentry_t *virt_to_xen_l3e(unsigned long v)
+{
+    l4_pgentry_t *pl4e;
+
+    pl4e = &idle_pg_table[l4_table_offset(v)];
+    if ( !(l4e_get_flags(*pl4e) & _PAGE_PRESENT) )
+    {
+        bool_t locking = system_state > SYS_STATE_boot;
+        l3_pgentry_t *pl3e = alloc_xen_pagetable();
+
+        if ( !pl3e )
+            return NULL;
+        clear_page(pl3e);
+        if ( locking )
+            spin_lock(&map_pgdir_lock);
+        if ( !(l4e_get_flags(*pl4e) & _PAGE_PRESENT) )
+        {
+            l4e_write(pl4e, l4e_from_paddr(__pa(pl3e), __PAGE_HYPERVISOR));
+            pl3e = NULL;
+        }
+        if ( locking )
+            spin_unlock(&map_pgdir_lock);
+        if ( pl3e )
+            free_xen_pagetable(pl3e);
+    }
+
+    return l4e_to_l3e(*pl4e) + l3_table_offset(v);
+}
+
+static l2_pgentry_t *virt_to_xen_l2e(unsigned long v)
+{
+    l3_pgentry_t *pl3e;
+
+    pl3e = virt_to_xen_l3e(v);
+    if ( !pl3e )
+        return NULL;
+
+    if ( !(l3e_get_flags(*pl3e) & _PAGE_PRESENT) )
+    {
+        bool_t locking = system_state > SYS_STATE_boot;
+        l2_pgentry_t *pl2e = alloc_xen_pagetable();
+
+        if ( !pl2e )
+            return NULL;
+        clear_page(pl2e);
+        if ( locking )
+            spin_lock(&map_pgdir_lock);
+        if ( !(l3e_get_flags(*pl3e) & _PAGE_PRESENT) )
+        {
+            l3e_write(pl3e, l3e_from_paddr(__pa(pl2e), __PAGE_HYPERVISOR));
+            pl2e = NULL;
+        }
+        if ( locking )
+            spin_unlock(&map_pgdir_lock);
+        if ( pl2e )
+            free_xen_pagetable(pl2e);
+    }
+
+    BUG_ON(l3e_get_flags(*pl3e) & _PAGE_PSE);
+    return l3e_to_l2e(*pl3e) + l2_table_offset(v);
+}
+
+l1_pgentry_t *virt_to_xen_l1e(unsigned long v)
+{
+    l2_pgentry_t *pl2e;
+
+    pl2e = virt_to_xen_l2e(v);
+    if ( !pl2e )
+        return NULL;
+
+    if ( !(l2e_get_flags(*pl2e) & _PAGE_PRESENT) )
+    {
+        bool_t locking = system_state > SYS_STATE_boot;
+        l1_pgentry_t *pl1e = alloc_xen_pagetable();
+
+        if ( !pl1e )
+            return NULL;
+        clear_page(pl1e);
+        if ( locking )
+            spin_lock(&map_pgdir_lock);
+        if ( !(l2e_get_flags(*pl2e) & _PAGE_PRESENT) )
+        {
+            l2e_write(pl2e, l2e_from_paddr(__pa(pl1e), __PAGE_HYPERVISOR));
+            pl1e = NULL;
+        }
+        if ( locking )
+            spin_unlock(&map_pgdir_lock);
+        if ( pl1e )
+            free_xen_pagetable(pl1e);
+    }
+
+    BUG_ON(l2e_get_flags(*pl2e) & _PAGE_PSE);
+    return l2e_to_l1e(*pl2e) + l1_table_offset(v);
+}
+
 /* Convert to from superpage-mapping flags for map_pages_to_xen(). */
 #define l1f_to_lNf(f) (((f) & _PAGE_PRESENT) ? ((f) |  _PAGE_PSE) : (f))
 #define lNf_to_l1f(f) (((f) & _PAGE_PRESENT) ? ((f) & ~_PAGE_PSE) : (f))
 
 /*
- * map_pages_to_xen() can be called with interrupts disabled:
- *  * During early bootstrap; or
- *  * alloc_xenheap_pages() via memguard_guard_range
- * In these cases it is safe to use flush_area_local():
- *  * Because only the local CPU is online; or
- *  * Because stale TLB entries do not matter for memguard_[un]guard_range().
+ * map_pages_to_xen() can be called with interrupts disabled during
+ * early bootstrap. In this case it is safe to use flush_area_local()
+ * and avoid locking because only the local CPU is online.
  */
 #define flush_area(v,f) (!local_irq_is_enabled() ?              \
                          flush_area_local((const void *)v, f) : \
@@ -5342,6 +5436,7 @@ int map_pages_to_xen(
     unsigned long nr_mfns,
     unsigned int flags)
 {
+    bool_t locking = system_state > SYS_STATE_boot;
     l2_pgentry_t *pl2e, ol2e;
     l1_pgentry_t *pl1e, ol1e;
     unsigned int  i;
@@ -5465,9 +5560,20 @@ int map_pages_to_xen(
             if ( l3e_get_flags(ol3e) & _PAGE_GLOBAL )
                 flush_flags |= FLUSH_TLB_GLOBAL;
 
-            l3e_write_atomic(pl3e, l3e_from_pfn(virt_to_mfn(pl2e),
-                                                __PAGE_HYPERVISOR));
+            if ( locking )
+                spin_lock(&map_pgdir_lock);
+            if ( (l3e_get_flags(*pl3e) & _PAGE_PRESENT) &&
+                 (l3e_get_flags(*pl3e) & _PAGE_PSE) )
+            {
+                l3e_write_atomic(pl3e, l3e_from_pfn(virt_to_mfn(pl2e),
+                                                    __PAGE_HYPERVISOR));
+                pl2e = NULL;
+            }
+            if ( locking )
+                spin_unlock(&map_pgdir_lock);
             flush_area(virt, flush_flags);
+            if ( pl2e )
+                free_xen_pagetable(pl2e);
         }
 
         pl2e = virt_to_xen_l2e(virt);
@@ -5559,9 +5665,20 @@ int map_pages_to_xen(
                 if ( l2e_get_flags(*pl2e) & _PAGE_GLOBAL )
                     flush_flags |= FLUSH_TLB_GLOBAL;
 
-                l2e_write_atomic(pl2e, l2e_from_pfn(virt_to_mfn(pl1e),
-                                                    __PAGE_HYPERVISOR));
+                if ( locking )
+                    spin_lock(&map_pgdir_lock);
+                if ( (l2e_get_flags(*pl2e) & _PAGE_PRESENT) &&
+                     (l2e_get_flags(*pl2e) & _PAGE_PSE) )
+                {
+                    l2e_write_atomic(pl2e, l2e_from_pfn(virt_to_mfn(pl1e),
+                                                        __PAGE_HYPERVISOR));
+                    pl1e = NULL;
+                }
+                if ( locking )
+                    spin_unlock(&map_pgdir_lock);
                 flush_area(virt, flush_flags);
+                if ( pl1e )
+                    free_xen_pagetable(pl1e);
             }
 
             pl1e  = l2e_to_l1e(*pl2e) + l1_table_offset(virt);
@@ -5587,7 +5704,10 @@ int map_pages_to_xen(
                     ((1 << PAGETABLE_ORDER) - 1)) == 0)) )
             {
                 unsigned long base_mfn;
+
                 pl1e = l2e_to_l1e(*pl2e);
+                if ( locking )
+                    spin_lock(&map_pgdir_lock);
                 base_mfn = l1e_get_pfn(*pl1e) & ~(L1_PAGETABLE_ENTRIES - 1);
                 for ( i = 0; i < L1_PAGETABLE_ENTRIES; i++, pl1e++ )
                     if ( (l1e_get_pfn(*pl1e) != (base_mfn + i)) ||
@@ -5598,11 +5718,15 @@ int map_pages_to_xen(
                     ol2e = *pl2e;
                     l2e_write_atomic(pl2e, l2e_from_pfn(base_mfn,
                                                         l1f_to_lNf(flags)));
+                    if ( locking )
+                        spin_unlock(&map_pgdir_lock);
                     flush_area(virt - PAGE_SIZE,
                                FLUSH_TLB_GLOBAL |
                                FLUSH_ORDER(PAGETABLE_ORDER));
                     free_xen_pagetable(l2e_to_l1e(ol2e));
                 }
+                else if ( locking )
+                    spin_unlock(&map_pgdir_lock);
             }
         }
 
@@ -5615,6 +5739,8 @@ int map_pages_to_xen(
         {
             unsigned long base_mfn;
 
+            if ( locking )
+                spin_lock(&map_pgdir_lock);
             ol3e = *pl3e;
             pl2e = l3e_to_l2e(ol3e);
             base_mfn = l2e_get_pfn(*pl2e) & ~(L2_PAGETABLE_ENTRIES *
@@ -5628,11 +5754,15 @@ int map_pages_to_xen(
             {
                 l3e_write_atomic(pl3e, l3e_from_pfn(base_mfn,
                                                     l1f_to_lNf(flags)));
+                if ( locking )
+                    spin_unlock(&map_pgdir_lock);
                 flush_area(virt - PAGE_SIZE,
                            FLUSH_TLB_GLOBAL |
                            FLUSH_ORDER(2*PAGETABLE_ORDER));
                 free_xen_pagetable(l3e_to_l2e(ol3e));
             }
+            else if ( locking )
+                spin_unlock(&map_pgdir_lock);
         }
     }
 
@@ -5641,6 +5771,7 @@ int map_pages_to_xen(
 
 void destroy_xen_mappings(unsigned long s, unsigned long e)
 {
+    bool_t locking = system_state > SYS_STATE_boot;
     l2_pgentry_t *pl2e;
     l1_pgentry_t *pl1e;
     unsigned int  i;
@@ -5679,8 +5810,19 @@ void destroy_xen_mappings(unsigned long s, unsigned long e)
                           l2e_from_pfn(l3e_get_pfn(*pl3e) +
                                        (i << PAGETABLE_ORDER),
                                        l3e_get_flags(*pl3e)));
-            l3e_write_atomic(pl3e, l3e_from_pfn(virt_to_mfn(pl2e),
-                                                __PAGE_HYPERVISOR));
+            if ( locking )
+                spin_lock(&map_pgdir_lock);
+            if ( (l3e_get_flags(*pl3e) & _PAGE_PRESENT) &&
+                 (l3e_get_flags(*pl3e) & _PAGE_PSE) )
+            {
+                l3e_write_atomic(pl3e, l3e_from_pfn(virt_to_mfn(pl2e),
+                                                    __PAGE_HYPERVISOR));
+                pl2e = NULL;
+            }
+            if ( locking )
+                spin_unlock(&map_pgdir_lock);
+            if ( pl2e )
+                free_xen_pagetable(pl2e);
         }
 
         pl2e = virt_to_xen_l2e(v);
@@ -5709,8 +5851,19 @@ void destroy_xen_mappings(unsigned long s, unsigned long e)
                     l1e_write(&pl1e[i],
                               l1e_from_pfn(l2e_get_pfn(*pl2e) + i,
                                            l2e_get_flags(*pl2e) & ~_PAGE_PSE));
-                l2e_write_atomic(pl2e, l2e_from_pfn(virt_to_mfn(pl1e),
-                                                    __PAGE_HYPERVISOR));
+                if ( locking )
+                    spin_lock(&map_pgdir_lock);
+                if ( (l2e_get_flags(*pl2e) & _PAGE_PRESENT) &&
+                     (l2e_get_flags(*pl2e) & _PAGE_PSE) )
+                {
+                    l2e_write_atomic(pl2e, l2e_from_pfn(virt_to_mfn(pl1e),
+                                                        __PAGE_HYPERVISOR));
+                    pl1e = NULL;
+                }
+                if ( locking )
+                    spin_unlock(&map_pgdir_lock);
+                if ( pl1e )
+                    free_xen_pagetable(pl1e);
             }
         }
         else