x86/P2M: synchronize fast and slow paths of p2m_get_page_from_gfn()

Handling of foreign pages was different between the two paths.

While permitting access to grants would be desirable, doing so would
require more involved handling; undo that for the time being. In
particular the page reference obtained would prevent the owning domain
from changing e.g. the page's type (after the grantee has released the
last reference of the grant). Instead perhaps another reference on the
grant would need obtaining. Which in turn would require determining
which grant that was.

Foreign pages in any event need permitting on both paths.

Introduce a helper function to be used on both paths, such that
respective checking differs in just the extra "to be unshared" condition
on the fast path.

While there adjust the sanity check for foreign pages: Don't leak the
reference on release builds when on a debug build the assertion would
have triggered. (Thanks to Roger for the suggestion.)

Fixes: 80ea7af172 ("x86/mm: Introduce get_page_from_gfn()")
Fixes: 50fe6e7370 ("pvh dom0: add and remove foreign pages")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
master commit: a8325f981c
master date: 2025-03-31 09:21:12 +0200

xen/arch/x86/mm/p2m.c

@@ -321,12 +321,46 @@ void p2m_put_gfn(struct p2m_domain *p2m, gfn_t gfn)
     gfn_unlock(p2m, gfn_x(gfn), 0);
 }
 
+static struct page_info *get_page_from_mfn_and_type(
+    const struct domain *d, mfn_t mfn, p2m_type_t t)
+{
+    struct page_info *page;
+
+    if ( !mfn_valid(mfn) )
+        return NULL;
+
+    page = mfn_to_page(mfn);
+
+    if ( p2m_is_ram(t) )
+    {
+        if ( p2m_is_shared(t) )
+            d = dom_cow;
+
+        if ( get_page(page, d) )
+            return page;
+    }
+    else if ( unlikely(p2m_is_foreign(t)) )
+    {
+        const struct domain *fdom = page_get_owner_and_reference(page);
+
+        if ( fdom )
+        {
+            if ( likely(fdom != d) )
+                return page;
+            ASSERT_UNREACHABLE();
+            put_page(page);
+        }
+    }
+
+    return NULL;
+}
+
 /* Atomically look up a GFN and take a reference count on the backing page. */
 struct page_info *p2m_get_page_from_gfn(
     struct p2m_domain *p2m, gfn_t gfn,
     p2m_type_t *t, p2m_access_t *a, p2m_query_t q)
 {
-    struct page_info *page = NULL;
+    struct page_info *page;
     p2m_access_t _a;
     p2m_type_t _t;
     mfn_t mfn;
@@ -340,26 +374,9 @@ struct page_info *p2m_get_page_from_gfn(
         /* Fast path: look up and get out */
         p2m_read_lock(p2m);
         mfn = p2m_get_gfn_type_access(p2m, gfn, t, a, 0, NULL, 0);
-        if ( p2m_is_any_ram(*t) && mfn_valid(mfn)
-             && !((q & P2M_UNSHARE) && p2m_is_shared(*t)) )
-        {
-            page = mfn_to_page(mfn);
-            if ( unlikely(p2m_is_foreign(*t)) )
-            {
-                struct domain *fdom = page_get_owner_and_reference(page);
-
-                ASSERT(fdom != p2m->domain);
-                if ( fdom == NULL )
-                    page = NULL;
-            }
-            else
-            {
-                struct domain *d = !p2m_is_shared(*t) ? p2m->domain : dom_cow;
-
-                if ( !get_page(page, d) )
-                    page = NULL;
-            }
-        }
+        page = !(q & P2M_UNSHARE) || !p2m_is_shared(*t)
+               ? get_page_from_mfn_and_type(p2m->domain, mfn, *t)
+               : NULL;
         p2m_read_unlock(p2m);
 
         if ( page )
@@ -373,14 +390,7 @@ struct page_info *p2m_get_page_from_gfn(
 
     /* Slow path: take the write lock and do fixups */
     mfn = get_gfn_type_access(p2m, gfn_x(gfn), t, a, q, NULL);
-    if ( p2m_is_ram(*t) && mfn_valid(mfn) )
-    {
-        struct domain *d = !p2m_is_shared(*t) ? p2m->domain : dom_cow;
-
-        page = mfn_to_page(mfn);
-        if ( !get_page(page, d) )
-            page = NULL;
-    }
+    page = get_page_from_mfn_and_type(p2m->domain, mfn, *t);
     put_gfn(p2m->domain, gfn_x(gfn));
 
     return page;