diff --git a/xen/common/hypfs.c b/xen/common/hypfs.c
index b74c228191..8e932b5cf9 100644
--- a/xen/common/hypfs.c
+++ b/xen/common/hypfs.c
@@ -297,7 +297,9 @@ int hypfs_write_leaf(struct hypfs_entry_leaf *leaf,
     int ret;
 
     ASSERT(this_cpu(hypfs_locked) == hypfs_write_locked);
-    ASSERT(ulen <= leaf->e.max_size);
+
+    if ( ulen > leaf->e.max_size )
+        return -ENOSPC;
 
     if ( leaf->e.type != XEN_HYPFS_TYPE_STRING &&
          leaf->e.type != XEN_HYPFS_TYPE_BLOB && ulen != leaf->e.size )
@@ -356,6 +358,10 @@ int hypfs_write_custom(struct hypfs_entry_leaf *leaf,
 
     ASSERT(this_cpu(hypfs_locked) == hypfs_write_locked);
 
+    /* Avoid oversized buffer allocation. */
+    if ( ulen > MAX_PARAM_SIZE )
+        return -ENOSPC;
+
     buf = xzalloc_array(char, ulen);
     if ( !buf )
         return -ENOMEM;
@@ -386,9 +392,6 @@ static int hypfs_write(struct hypfs_entry *entry,
 
     ASSERT(entry->max_size);
 
-    if ( ulen > entry->max_size )
-        return -ENOSPC;
-
     l = container_of(entry, struct hypfs_entry_leaf, e);
 
     return entry->write(l, uaddr, ulen);
diff --git a/xen/common/kernel.c b/xen/common/kernel.c
index 9de07b7ac5..c3a943f077 100644
--- a/xen/common/kernel.c
+++ b/xen/common/kernel.c
@@ -57,7 +57,7 @@ static int assign_integer_param(const struct kernel_param *param, uint64_t val)
 static int parse_params(const char *cmdline, const struct kernel_param *start,
                         const struct kernel_param *end)
 {
-    char opt[128], *optval, *optkey, *q;
+    char opt[MAX_PARAM_SIZE], *optval, *optkey, *q;
     const char *p = cmdline, *key;
     const struct kernel_param *param;
     int rc, final_rc = 0;
diff --git a/xen/include/xen/param.h b/xen/include/xen/param.h
index f4be944248..d0409d3a0e 100644
--- a/xen/include/xen/param.h
+++ b/xen/include/xen/param.h
@@ -26,6 +26,9 @@ struct kernel_param {
     } par;
 };
 
+/* Maximum length of a single parameter string. */
+#define MAX_PARAM_SIZE 128
+
 extern const struct kernel_param __setup_start[], __setup_end[];
 
 #define __param(att)      static const att \