userns: Changing any namespace id mappings should require privileges

Changing uid/gid/projid mappings doesn't change your id within the
namespace; it reconfigures the namespace.  Unprivileged programs should
*not* be able to write these files.  (We're also checking the privileges
on the wrong task.)

Given the write-once nature of these files and the other security
checks, this is likely impossible to usefully exploit.

Signed-off-by: Andy Lutomirski <>
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 5c16f3a..e134d8f 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -613,10 +613,10 @@
 	if (map->nr_extents != 0)
 		goto out;
-	/* Require the appropriate privilege CAP_SETUID or CAP_SETGID
-	 * over the user namespace in order to set the id mapping.
+	/*
+	 * Adjusting namespace settings requires capabilities on the target.
-	if (cap_valid(cap_setid) && !ns_capable(ns, cap_setid))
+	if (cap_valid(cap_setid) && !file_ns_capable(file, ns, CAP_SYS_ADMIN))
 		goto out;
 	/* Get a buffer */