memory: fix return value handing of guest_remove_page()

Despite the description in mm.h, guest_remove_page() previously returned 0 for
paging errors.

Switch guest_remove_page() to having regular 0/-error semantics, and propagate
the return values from clear_mmio_p2m_entry() and mem_sharing_unshare_page()
to the callers (although decrease_reservation() is the only caller which
currently cares).

This is part of XSA-222.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: b614f642c3
master date: 2017-06-20 14:39:56 +0200

xen/common/memory.c

@@ -238,6 +238,7 @@ int guest_remove_page(struct domain *d, unsigned long gmfn)
     p2m_type_t p2mt;
 #endif
     unsigned long mfn;
+    int rc;
 
 #ifdef CONFIG_X86
     mfn = mfn_x(get_gfn_query(d, gmfn, &p2mt)); 
@@ -255,13 +256,15 @@ int guest_remove_page(struct domain *d, unsigned long gmfn)
                 put_page(page);
         }
         p2m_mem_paging_drop_page(d, gmfn, p2mt);
-        return 1;
+
+        return 0;
     }
     if ( p2mt == p2m_mmio_direct )
     {
-        clear_mmio_p2m_entry(d, gmfn, _mfn(mfn));
+        rc = clear_mmio_p2m_entry(d, gmfn, _mfn(mfn));
         put_gfn(d, gmfn);
-        return 1;
+
+        return rc;
     }
 #else
     mfn = gmfn_to_mfn(d, gmfn);
@@ -271,21 +274,25 @@ int guest_remove_page(struct domain *d, unsigned long gmfn)
         put_gfn(d, gmfn);
         gdprintk(XENLOG_INFO, "Domain %u page number %lx invalid\n",
                 d->domain_id, gmfn);
-        return 0;
+
+        return -EINVAL;
     }
             
 #ifdef CONFIG_X86
     if ( p2m_is_shared(p2mt) )
     {
-        /* Unshare the page, bail out on error. We unshare because 
-         * we might be the only one using this shared page, and we
-         * need to trigger proper cleanup. Once done, this is 
-         * like any other page. */
-        if ( mem_sharing_unshare_page(d, gmfn, 0) )
+        /*
+         * Unshare the page, bail out on error. We unshare because we
+         * might be the only one using this shared page, and we need to
+         * trigger proper cleanup. Once done, this is like any other page.
+         */
+        rc = mem_sharing_unshare_page(d, gmfn, 0);
+        if ( rc )
         {
             put_gfn(d, gmfn);
             (void)mem_sharing_notify_enomem(d, gmfn, 0);
-            return 0;
+
+            return rc;
         }
         /* Maybe the mfn changed */
         mfn = mfn_x(get_gfn_query_unlocked(d, gmfn, &p2mt));
@@ -298,7 +305,8 @@ int guest_remove_page(struct domain *d, unsigned long gmfn)
     {
         put_gfn(d, gmfn);
         gdprintk(XENLOG_INFO, "Bad page free for domain %u\n", d->domain_id);
-        return 0;
+
+        return -ENXIO;
     }
 
     if ( test_and_clear_bit(_PGT_pinned, &page->u.inuse.type_info) )
@@ -312,7 +320,7 @@ int guest_remove_page(struct domain *d, unsigned long gmfn)
     put_page(page);
     put_gfn(d, gmfn);
 
-    return 1;
+    return 0;
 }
 
 static void decrease_reservation(struct memop_args *a)
@@ -363,7 +371,7 @@ static void decrease_reservation(struct memop_args *a)
             continue;
 
         for ( j = 0; j < (1 << a->extent_order); j++ )
-            if ( !guest_remove_page(a->domain, gmfn + j) )
+            if ( guest_remove_page(a->domain, gmfn + j) )
                 goto out;
     }
 

xen/include/xen/mm.h

@@ -381,8 +381,7 @@ int xenmem_add_to_physmap_one(struct domain *d, unsigned int space,
                               domid_t foreign_domid,
                               unsigned long idx, xen_pfn_t gpfn);
 
-/* Returns 1 on success, 0 on error, negative if the ring
- * for event propagation is full in the presence of paging */
+/* Returns 0 on success, or negative on error. */
 int guest_remove_page(struct domain *d, unsigned long gmfn);
 
 #define RAM_TYPE_CONVENTIONAL 0x00000001