steal_page: Get rid of bogus struct page states

The original rules for `struct page` required the following invariants
at all times:

- refcount > 0 implies owner != NULL
- PGC_allocated implies refcount > 0

steal_page, in a misguided attempt to protect against unknown races,
violates both of these rules, thus introducing other races:

- Temporarily, the count_info has the refcount go to 0 while
  PGC_allocated is set

- It explicitly returns the page PGC_allocated set, but owner == NULL
  and page not on the page_list.

The second one meant that page_get_owner_and_reference() could return
NULL even after having successfully grabbed a reference on the page,
leading the caller to leak the reference (since "couldn't get ref" and
"got ref but no owner" look the same).

Furthermore, rather than grabbing a page reference to ensure that the
owner doesn't change under its feet, it appears to rely on holding
d->page_alloc lock to prevent this.

Unfortunately, this is ineffective: page->owner remains non-NULL for
some time after the count has been set to 0; meaning that it would be
entirely possible for the page to be freed and re-allocated to a
different domain between the page_get_owner() check and the count_info
check.

Modify steal_page to instead follow the appropriate access discipline,
taking the page through series of states similar to being freed and
then re-allocated with MEMF_no_owner:

- Grab an extra reference to make sure we don't race with anyone else
  freeing the page

- Drop both references and PGC_allocated atomically, so that (if
successful), anyone else trying to grab a reference will fail

- Attempt to reset Xen's mappings

- Reset the rest of the state.

Then, modify the two callers appropriately:

- Leave count_info alone (it's already been cleared)
- Call free_domheap_page() directly if appropriate
- Call assign_pages() rather than open-coding a partial assign

With all callers to assign_pages() now passing in pages with the
type_info field clear, tighten the respective assertion there.

This is XSA-287.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/mm.c

@@ -3989,70 +3989,106 @@ int donate_page(
     return -EINVAL;
 }
 
+/*
+ * Steal page will attempt to remove `page` from domain `d`.  Upon
+ * return, `page` will be in a state similar to the state of a page
+ * returned from alloc_domheap_page() with MEMF_no_owner set:
+ * - refcount 0
+ * - type count cleared
+ * - owner NULL
+ * - page caching attributes cleaned up
+ * - removed from the domain's page_list
+ *
+ * If MEMF_no_refcount is not set, the domain's tot_pages will be
+ * adjusted.  If this results in the page count falling to 0,
+ * put_domain() will be called.
+ *
+ * The caller should either call free_domheap_page() to free the
+ * page, or assign_pages() to put it back on some domain's page list.
+ */
 int steal_page(
     struct domain *d, struct page_info *page, unsigned int memflags)
 {
     unsigned long x, y;
     bool drop_dom_ref = false;
-    const struct domain *owner = dom_xen;
+    const struct domain *owner;
+    int rc;
 
     if ( paging_mode_external(d) )
         return -EOPNOTSUPP;
 
-    spin_lock(&d->page_alloc_lock);
-
-    if ( is_xen_heap_page(page) || ((owner = page_get_owner(page)) != d) )
+    /* Grab a reference to make sure the page doesn't change under our feet */
+    rc = -EINVAL;
+    if ( !(owner = page_get_owner_and_reference(page)) )
         goto fail;
 
+    if ( owner != d || is_xen_heap_page(page) )
+        goto fail_put;
+
     /*
-     * We require there is just one reference (PGC_allocated). We temporarily
-     * drop this reference now so that we can safely swizzle the owner.
+     * We require there are exactly two references -- the one we just
+     * took, and PGC_allocated. We temporarily drop both these
+     * references so that the page becomes effectively non-"live" for
+     * the domain.
      */
     y = page->count_info;
     do {
         x = y;
-        if ( (x & (PGC_count_mask|PGC_allocated)) != (1 | PGC_allocated) )
-            goto fail;
-        y = cmpxchg(&page->count_info, x, x & ~PGC_count_mask);
+        if ( (x & (PGC_count_mask|PGC_allocated)) != (2 | PGC_allocated) )
+            goto fail_put;
+        y = cmpxchg(&page->count_info, x, x & ~(PGC_count_mask|PGC_allocated));
     } while ( y != x );
 
     /*
-     * With the sole reference dropped temporarily, no-one can update type
-     * information. Type count also needs to be zero in this case, but e.g.
-     * PGT_seg_desc_page may still have PGT_validated set, which we need to
-     * clear before transferring ownership (as validation criteria vary
-     * depending on domain type).
+     * NB this is safe even if the page ends up being given back to
+     * the domain, because the count is zero: subsequent mappings will
+     * cause the cache attributes to be re-instated inside
+     * get_page_from_l1e().
      */
+    if ( (rc = cleanup_page_cacheattr(page)) )
+    {
+        /*
+         * Couldn't fixup Xen's mappings; put things the way we found
+         * it and return an error
+         */
+        page->count_info |= PGC_allocated | 1;
+        goto fail;
+    }
+
+    /*
+     * With the reference count now zero, nobody can grab references
+     * to do anything else with the page.  Return the page to a state
+     * that it might be upon return from alloc_domheap_pages with
+     * MEMF_no_owner set.
+     */
+    spin_lock(&d->page_alloc_lock);
+
     BUG_ON(page->u.inuse.type_info & (PGT_count_mask | PGT_locked |
                                       PGT_pinned));
     page->u.inuse.type_info = 0;
-
-    /* Swizzle the owner then reinstate the PGC_allocated reference. */
     page_set_owner(page, NULL);
-    y = page->count_info;
-    do {
-        x = y;
-        BUG_ON((x & (PGC_count_mask|PGC_allocated)) != PGC_allocated);
-    } while ( (y = cmpxchg(&page->count_info, x, x | 1)) != x );
+    page_list_del(page, &d->page_list);
 
     /* Unlink from original owner. */
     if ( !(memflags & MEMF_no_refcount) && !domain_adjust_tot_pages(d, -1) )
         drop_dom_ref = true;
-    page_list_del(page, &d->page_list);
 
     spin_unlock(&d->page_alloc_lock);
+
     if ( unlikely(drop_dom_ref) )
         put_domain(d);
+
     return 0;
 
+ fail_put:
+    put_page(page);
  fail:
-    spin_unlock(&d->page_alloc_lock);
     gdprintk(XENLOG_WARNING, "Bad steal mfn %" PRI_mfn
              " from d%d (owner d%d) caf=%08lx taf=%" PRtype_info "\n",
              mfn_x(page_to_mfn(page)), d->domain_id,
              owner ? owner->domain_id : DOMID_INVALID,
              page->count_info, page->u.inuse.type_info);
-    return -EINVAL;
+    return rc;
 }
 
 #ifdef CONFIG_PV

xen/common/grant_table.c

@@ -2157,7 +2157,7 @@ gnttab_transfer(
 #ifdef CONFIG_X86
             put_gfn(d, gop.mfn);
 #endif
-            page->count_info &= ~(PGC_count_mask|PGC_allocated);
+            /* The count_info has already been cleaned */
             free_domheap_page(page);
             goto copyback;
         }
@@ -2180,10 +2180,9 @@ gnttab_transfer(
 
             copy_domain_page(page_to_mfn(new_page), mfn);
 
-            page->count_info &= ~(PGC_count_mask|PGC_allocated);
+            /* The count_info has already been cleared */
             free_domheap_page(page);
             page = new_page;
-            page->count_info = PGC_allocated | 1;
             mfn = page_to_mfn(page);
         }
 
@@ -2223,12 +2222,17 @@ gnttab_transfer(
          */
         spin_unlock(&e->page_alloc_lock);
         okay = gnttab_prepare_for_transfer(e, d, gop.ref);
-        spin_lock(&e->page_alloc_lock);
 
-        if ( unlikely(!okay) || unlikely(e->is_dying) )
+        if ( unlikely(!okay || assign_pages(e, page, 0, MEMF_no_refcount)) )
         {
-            bool_t drop_dom_ref = !domain_adjust_tot_pages(e, -1);
+            bool drop_dom_ref;
 
+            /*
+             * Need to grab this again to safely free our "reserved"
+             * page in the page total
+             */
+            spin_lock(&e->page_alloc_lock);
+            drop_dom_ref = !domain_adjust_tot_pages(e, -1);
             spin_unlock(&e->page_alloc_lock);
 
             if ( okay /* i.e. e->is_dying due to the surrounding if() */ )
@@ -2241,10 +2245,6 @@ gnttab_transfer(
             goto unlock_and_copyback;
         }
 
-        page_list_add_tail(page, &e->page_list);
-        page_set_owner(page, e);
-
-        spin_unlock(&e->page_alloc_lock);
 #ifdef CONFIG_X86
         put_gfn(d, gop.mfn);
 #endif

xen/common/memory.c

@@ -694,20 +694,22 @@ static long memory_exchange(XEN_GUEST_HANDLE_PARAM(xen_memory_exchange_t) arg)
          * Success! Beyond this point we cannot fail for this chunk.
          */
 
-        /* Destroy final reference to each input page. */
+        /*
+         * These pages have already had owner and reference cleared.
+         * Do the final two steps: Remove from the physmap, and free
+         * them.
+         */
         while ( (page = page_list_remove_head(&in_chunk_list)) )
         {
             unsigned long gfn;
 
-            if ( !test_and_clear_bit(_PGC_allocated, &page->count_info) )
-                BUG();
             mfn = page_to_mfn(page);
             gfn = mfn_to_gmfn(d, mfn_x(mfn));
             /* Pages were unshared above */
             BUG_ON(SHARED_M2P(gfn));
             if ( guest_physmap_remove_page(d, _gfn(gfn), mfn, 0) )
                 domain_crash(d);
-            put_page(page);
+            free_domheap_page(page);
         }
 
         /* Assign each output page to the domain. */
@@ -780,13 +782,16 @@ static long memory_exchange(XEN_GUEST_HANDLE_PARAM(xen_memory_exchange_t) arg)
      * chunks succeeded.
      */
  fail:
-    /* Reassign any input pages we managed to steal. */
+    /*
+     * Reassign any input pages we managed to steal.  NB that if the assign
+     * fails again, we're on the hook for freeing the page, since we've already
+     * cleared PGC_allocated.
+     */
     while ( (page = page_list_remove_head(&in_chunk_list)) )
         if ( assign_pages(d, page, 0, MEMF_no_refcount) )
         {
             BUG_ON(!d->is_dying);
-            if ( test_and_clear_bit(_PGC_allocated, &page->count_info) )
-                put_page(page);
+            free_domheap_page(page);
         }
 
  dying:

xen/common/page_alloc.c

@@ -2279,7 +2279,7 @@ int assign_pages(
     for ( i = 0; i < (1 << order); i++ )
     {
         ASSERT(page_get_owner(&pg[i]) == NULL);
-        ASSERT((pg[i].count_info & ~(PGC_allocated | 1)) == 0);
+        ASSERT(!pg[i].count_info);
         page_set_owner(&pg[i], d);
         smp_wmb(); /* Domain pointer must be visible before updating refcnt. */
         pg[i].count_info = PGC_allocated | 1;