xen/gnttab: Prevent GNTTABOP_cache_flush from corrupting RO grants

Grants can be offered read-only by the owning domain.  While a domain mapping
a RO grant may have a need to invalidate the cache, it must not modify the
other-guests data while doing so.  Force-upgrade to a clean operation in this
case.

This is CVE-2026-102702, part of XSA-519.

Fixes: 18e8d22fe7 ("introduce GNTTABOP_cache_flush")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
(cherry picked from commit c89185286c)

xen/common/grant_table.c

@@ -983,6 +983,13 @@ static struct active_grant_entry *grant_map_exists(const struct domain *ld,
     return ERR_PTR(-EINVAL);
 }
 
+static bool grant_map_is_ro(struct active_grant_entry *act)
+{
+    ASSERT(spin_is_locked(&act->lock));
+
+    return (act->pin & (GNTPIN_hstw_mask | GNTPIN_devw_mask)) == 0;
+}
+
 union maptrack_node {
     struct {
         /* Radix tree slot pointers use two of the bits. */
@@ -3590,7 +3597,7 @@ gnttab_swap_grant_ref(XEN_GUEST_HANDLE_PARAM(gnttab_swap_grant_ref_t) uop,
     return 0;
 }
 
-static int _cache_flush(const gnttab_cache_flush_t *cflush, grant_ref_t *cur_ref)
+static int _cache_flush(gnttab_cache_flush_t *cflush, grant_ref_t *cur_ref)
 {
     struct domain *d, *owner;
     struct page_info *page;
@@ -3641,6 +3648,24 @@ static int _cache_flush(const gnttab_cache_flush_t *cflush, grant_ref_t *cur_ref
             put_page(page);
             return act ? PTR_ERR(act) : 1;
         }
+
+        /*
+         * If the grant mapping is read-only, it must be cleaned as well as
+         * invalidated.  Invalidation without cleaning may modify memory the
+         * granter is expecting not to change.
+         *
+         * It is highly unlikely but legal to have multiple grant mappings of
+         * differing read-only-ness.  If a read-write grant mapping is found
+         * first, then it's fine to invalidate without clean.
+         *
+         * If a read-only grant mapping is found first, a later read-write
+         * mapping could be missed.  Assume that the read-only mapping is the
+         * only mapping.  The cost is a clean of up to 4k, while the saving is
+         * not requiring the search of grant maps to be exhaustive in order to
+         * spot a corner-case expected not to exist in practice.
+         */
+        if ( grant_map_is_ro(act) )
+            cflush->op |= GNTTAB_CACHE_CLEAN;
     }
 
     v = map_domain_page(mfn);