selftests: cgroup: Test open-time credential usage for migration checks

When a task is writing to an fd opened by a different task, the perm check
should use the credentials of the latter task. Add a test for it.

Tested-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/tools/testing/selftests/cgroup/test_core.c b/tools/testing/selftests/cgroup/test_core.c
index 3df648c378765653348791d7f4e53d20b6ee3eb5..01b76650697359ed863b19fe9522048aa9822ebf 100644 (file)
--- a/tools/testing/selftests/cgroup/test_core.c
+++ b/tools/testing/selftests/cgroup/test_core.c
@@ -674,6 +674,73 @@ cleanup:
        return ret;
 }
 
+/*
+ * cgroup migration permission check should be performed based on the
+ * credentials at the time of open instead of write.
+ */
+static int test_cgcore_lesser_euid_open(const char *root)
+{
+       const uid_t test_euid = 65534;  /* usually nobody, any !root is fine */
+       int ret = KSFT_FAIL;
+       char *cg_test_a = NULL, *cg_test_b = NULL;
+       char *cg_test_a_procs = NULL, *cg_test_b_procs = NULL;
+       int cg_test_b_procs_fd = -1;
+       uid_t saved_uid;
+
+       cg_test_a = cg_name(root, "cg_test_a");
+       cg_test_b = cg_name(root, "cg_test_b");
+
+       if (!cg_test_a || !cg_test_b)
+               goto cleanup;
+
+       cg_test_a_procs = cg_name(cg_test_a, "cgroup.procs");
+       cg_test_b_procs = cg_name(cg_test_b, "cgroup.procs");
+
+       if (!cg_test_a_procs || !cg_test_b_procs)
+               goto cleanup;
+
+       if (cg_create(cg_test_a) || cg_create(cg_test_b))
+               goto cleanup;
+
+       if (cg_enter_current(cg_test_a))
+               goto cleanup;
+
+       if (chown(cg_test_a_procs, test_euid, -1) ||
+           chown(cg_test_b_procs, test_euid, -1))
+               goto cleanup;
+
+       saved_uid = geteuid();
+       if (seteuid(test_euid))
+               goto cleanup;
+
+       cg_test_b_procs_fd = open(cg_test_b_procs, O_RDWR);
+
+       if (seteuid(saved_uid))
+               goto cleanup;
+
+       if (cg_test_b_procs_fd < 0)
+               goto cleanup;
+
+       if (write(cg_test_b_procs_fd, "0", 1) >= 0 || errno != EACCES)
+               goto cleanup;
+
+       ret = KSFT_PASS;
+
+cleanup:
+       cg_enter_current(root);
+       if (cg_test_b_procs_fd >= 0)
+               close(cg_test_b_procs_fd);
+       if (cg_test_b)
+               cg_destroy(cg_test_b);
+       if (cg_test_a)
+               cg_destroy(cg_test_a);
+       free(cg_test_b_procs);
+       free(cg_test_a_procs);
+       free(cg_test_b);
+       free(cg_test_a);
+       return ret;
+}
+
 #define T(x) { x, #x }
 struct corecg_test {
        int (*fn)(const char *root);
@@ -689,6 +756,7 @@ struct corecg_test {
        T(test_cgcore_proc_migration),
        T(test_cgcore_thread_migration),
        T(test_cgcore_destroy),
+       T(test_cgcore_lesser_euid_open),
 };
 #undef T