[PATCH]Fix kernel NULL function pointer dereference in vfs_kern_mount()

[Dennis Chen]

This patch is try to fix a potential NULL function pointer dereference in the exported
vfs_kern_mount() function which can be triggered by following kernel module code piece:

static struct vfsmount *npfs_mnt;
static struct file_system_type np_fs_type = {
        .name		= "npfs",
        .kill_sb      	= npfs_kill_sb,
        .fs_flags     	= FS_USERNS_MOUNT,
};

static int fsmod_init(void)
{
	int err = -ENOMEM;

	err = register_filesystem(&np_fs_type);
	if (!err) {
		npfs_mnt = kern_mount(&np_fs_type);
		if (IS_ERR(npfs_mnt)) {
			printk(KERN_ERR "npfs: could not mount!\n");
			unregister_filesystem(&np_fs_type);
			return PTR_ERR(npfs_mnt);
		}
	}

	return err;
}

I happened to forget to implement a (*mount)() function in the np_fs_type instance which
is buggy definitely, but as an exported function to the outside, kernel should not suppose
the caller will always take the right action. In this scenario, I have to reboot my machine
to clean the loaded kernel module.

Signed-off-by: Dennis Chen <xschen@xxxxxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
---
fs/filesystems.c |    5 +++++
fs/namespace.c   |    2 ++
fs/super.c       |   10 ++++++++++
3 files changed, 17 insertions(+)

diff --git a/fs/filesystems.c b/fs/filesystems.c
index 92567d9..6d6b162 100644
--- a/fs/filesystems.c
+++ b/fs/filesystems.c
@@ -71,6 +71,11 @@ int register_filesystem(struct file_system_type * fs)
        int res = 0;
        struct file_system_type ** p;
 
+       if (!fs)
+               return -ENODEV;
+       if (!fs->name)
+               return -EINVAL;
+
        BUG_ON(strchr(fs->name, '.'));
        if (fs->next)
                return -EBUSY;
diff --git a/fs/namespace.c b/fs/namespace.c
index 7b1ca9b..ad17692 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2758,6 +2758,8 @@ void put_mnt_ns(struct mnt_namespace *ns)
 struct vfsmount *kern_mount_data(struct file_system_type *type, void *data)
 {
        struct vfsmount *mnt;
+       if (!type)
+               return ERR_PTR(-ENODEV);
        mnt = vfs_kern_mount(type, MS_KERNMOUNT, type->name, data);
        if (!IS_ERR(mnt)) {
                /*
diff --git a/fs/super.c b/fs/super.c
index 68307c0..d995a19 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1101,11 +1101,21 @@ mount_fs(struct file_system_type *type, int flags, const char *name, void *data)
                        goto out_free_secdata;
        }
 
+       if (!type->mount) {
+               error = -EINVAL;
+               printk(KERN_ERR "No 'mount' method defined in %s\n", type->name);
+               goto out_free_secdata;
+       }
        root = type->mount(type, flags, name, data);
        if (IS_ERR(root)) {
                error = PTR_ERR(root);
                goto out_free_secdata;
        }
+       if (!root) {
+               error = -EINVAL;
+               goto out_free_secdata;
+       }
+
        sb = root->d_sb;
        BUG_ON(!sb);
        WARN_ON(!sb->s_bdi);

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