cgroup: initialize xattr before calling d_instantiate()

cgroup_create_file() calls d_instantiate(), which may decide to look
at the xattrs on the file. Smack always does this and SELinux can be
configured to do so.

But cgroup_add_file() didn't initialize xattrs before calling
cgroup_create_file(), which finally leads to dereferencing NULL
dentry->d_fsdata.

This bug has been there since cgroup xattr was introduced.

Cc: <stable@vger.kernel.org> # 3.8.x
Reported-by: Ivan Bulatovic <combuster@archlinux.us>
Reported-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 5 insertions, 4 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 2a9926275f80..38b136553044 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2699,13 +2699,14 @@ static int cgroup_add_file(struct cgroup *cgrp, struct cgroup_subsys *subsys,
                 goto out;
         }
 
+        cfe->type = (void *)cft;
+        cfe->dentry = dentry;
+        dentry->d_fsdata = cfe;
+        simple_xattrs_init(&cfe->xattrs);
+
         mode = cgroup_file_mode(cft);
         error = cgroup_create_file(dentry, mode | S_IFREG, cgrp->root->sb);
         if (!error) {
-                cfe->type = (void *)cft;
-                cfe->dentry = dentry;
-                dentry->d_fsdata = cfe;
-                simple_xattrs_init(&cfe->xattrs);
                 list_add_tail(&cfe->node, &parent->files);
                 cfe = NULL;
         }