gnttab: split gnttab_map_frame()

If a domain tries to map status frames in parallel to switching grant
table version from 2 to 1, the mapping operation may put in place P2M
entries referencing MFNs which gnttab_unpopulate_status_frames() is in the
process of freeing.

Ideally we would refcount pages when entered into P2M tables, but that's a
significant change. Extend the grant-table-locked region instead in
xenmem_add_to_physmap_one() (being the sole caller of gnttab_map_frame()),
such that a race with gnttab_unpopulate_status_frames() is no longer
possible.

This is XSA-486 / CVE-2026-23558.

Fixes: 5ce8fafa94 ("Dynamic grant-table sizing")
Fixes: a98dc13703 ("Introduce a grant_entry_v2 structure")
Reported-by: Rafal Wojtczuk <rafal.wojtczuk@7bulls.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
(cherry picked from commit 02708d3b51)

xen/arch/arm/mm.c

@@ -114,12 +114,10 @@ int xenmem_add_to_physmap_one(
     switch ( space )
     {
     case XENMAPSPACE_grant_table:
-        rc = gnttab_map_frame(d, idx, gfn, &mfn);
+        rc = gnttab_map_frame_begin(d, idx, gfn, &mfn);
         if ( rc )
             return rc;
 
-        /* Need to take care of the reference obtained in gnttab_map_frame(). */
-        page = mfn_to_page(mfn);
         t = p2m_ram_rw;
 
         break;
@@ -221,10 +219,23 @@ int xenmem_add_to_physmap_one(
      * to drop the reference we took earlier. In all other cases we need to
      * drop any reference we took earlier (perhaps indirectly).
      */
-    if ( space == XENMAPSPACE_gmfn_foreign ? rc : page != NULL )
+    switch ( space )
     {
+    default:
+        if ( page )
+            put_page(page);
+        break;
+
+    case XENMAPSPACE_grant_table:
+        gnttab_map_frame_end(d, mfn);
+        break;
+
+    case XENMAPSPACE_gmfn_foreign:
+        if ( !rc )
+            break;
         ASSERT(page != NULL);
         put_page(page);
+        break;
     }
 
     return rc;

xen/arch/x86/mm/p2m.c

@@ -1984,11 +1984,9 @@ int xenmem_add_to_physmap_one(
         break;
 
     case XENMAPSPACE_grant_table:
-        rc = gnttab_map_frame(d, idx, gfn, &mfn);
+        rc = gnttab_map_frame_begin(d, idx, gfn, &mfn);
         if ( rc )
             return rc;
-        /* Need to take care of the reference obtained in gnttab_map_frame(). */
-        page = mfn_to_page(mfn);
         break;
 
     case XENMAPSPACE_gmfn:
@@ -2070,19 +2068,28 @@ int xenmem_add_to_physmap_one(
     put_gfn(d, gfn_x(gfn));
 
  put_both:
-    /*
-     * In the XENMAPSPACE_gmfn case, we took a ref of the gfn at the top.
-     * We also may need to transfer ownership of the page reference to our
-     * caller.
-     */
-    if ( space == XENMAPSPACE_gmfn )
+    switch ( space )
     {
+    case XENMAPSPACE_gmfn:
+        /*
+         * We took a ref of the gfn at the top.  We also may need to transfer
+         * ownership of the page reference to our caller.
+         */
         put_gfn(d, gmfn);
         if ( !rc && extra.ppage )
         {
             *extra.ppage = page;
             page = NULL;
         }
+        break;
+
+    case XENMAPSPACE_grant_table:
+        /*
+         * We (gnttab_map_frame_begin()) acquired a lock and took a ref of the
+         * page underlying the MFN at the top.
+         */
+        gnttab_map_frame_end(d, mfn);
+        break;
     }
 
     if ( page )

xen/common/grant_table.c

@@ -4242,7 +4242,8 @@ int gnttab_acquire_resource(
     return rc;
 }
 
-int gnttab_map_frame(struct domain *d, unsigned long idx, gfn_t gfn, mfn_t *mfn)
+int gnttab_map_frame_begin(
+    struct domain *d, unsigned long idx, gfn_t gfn, mfn_t *mfn)
 {
     int rc = 0;
     struct grant_table *gt = d->grant_table;
@@ -4280,11 +4281,19 @@ int gnttab_map_frame(struct domain *d, unsigned long idx, gfn_t gfn, mfn_t *mfn)
             put_page(pg);
     }
 
-    grant_write_unlock(gt);
+    if ( rc )
+        grant_write_unlock(d->grant_table);
 
     return rc;
 }
 
+void gnttab_map_frame_end(struct domain *d, mfn_t mfn)
+{
+    put_page(mfn_to_page(mfn));
+
+    grant_write_unlock(d->grant_table);
+}
+
 static void gnttab_usage_print(struct domain *rd)
 {
     int first = 1;

xen/include/xen/grant_table.h

@@ -56,8 +56,13 @@ int gnttab_release_mappings(struct domain *d);
 int mem_sharing_gref_to_gfn(struct grant_table *gt, grant_ref_t ref,
                             gfn_t *gfn, uint16_t *status);
 
-int gnttab_map_frame(struct domain *d, unsigned long idx, gfn_t gfn,
-                     mfn_t *mfn);
+/*
+ * These need to be used as a pair, as the first (in the success case) returns
+ * with a lock and page reference held which the second needs to drop.
+ */
+int gnttab_map_frame_begin(struct domain *d, unsigned long idx, gfn_t gfn,
+                           mfn_t *mfn);
+void gnttab_map_frame_end(struct domain *d, mfn_t mfn);
 
 unsigned int gnttab_resource_max_frames(const struct domain *d, unsigned int id);
 
@@ -96,12 +101,14 @@ static inline int mem_sharing_gref_to_gfn(struct grant_table *gt,
     return -EINVAL;
 }
 
-static inline int gnttab_map_frame(struct domain *d, unsigned long idx,
-                                   gfn_t gfn, mfn_t *mfn)
+static inline int gnttab_map_frame_begin(struct domain *d, unsigned long idx,
+                                         gfn_t gfn, mfn_t *mfn)
 {
     return -EINVAL;
 }
 
+static inline void gnttab_map_frame_end(struct domain *d, mfn_t mfn) {}
+
 static inline unsigned int gnttab_resource_max_frames(
     const struct domain *d, unsigned int id)
 {