[PATCH v4] vfs, security: Fix automount superblock LSM init problem, preventing NFS sb sharing

[Al Viro]

On Wed, Aug 24, 2022 at 11:09:50AM +0100, David Howells wrote:

What's the reason for difference between selinux and smack instances of
context_init?  The former allocates only on submount, the latter -
unconditionally...

> +static int selinux_fs_context_init(struct fs_context *fc,
> +				   struct dentry *reference)
> +{
> +	const struct superblock_security_struct *sbsec;
> +	const struct inode_security_struct *root_isec;
> +	struct selinux_mnt_opts *opts;
> +
> +	if (fc->purpose == FS_CONTEXT_FOR_SUBMOUNT) {
> +		opts = kzalloc(sizeof(*opts), GFP_KERNEL);
> +		if (!opts)
> +			return -ENOMEM;
> +
> +		root_isec = backing_inode_security(reference->d_sb->s_root);
> +		sbsec = selinux_superblock(reference->d_sb);
> +		if (sbsec->flags & FSCONTEXT_MNT)
> +			opts->fscontext_sid	= sbsec->sid;
> +		if (sbsec->flags & CONTEXT_MNT)
> +			opts->context_sid	= sbsec->mntpoint_sid;
> +		if (sbsec->flags & DEFCONTEXT_MNT)
> +			opts->defcontext_sid	= sbsec->def_sid;
> +		fc->security = opts;
> +	}
> +
> +	return 0;
> +}

> +/**
> + * smack_fs_context_init - Initialise security data for a filesystem context
> + * @fc: The filesystem context.
> + * @reference: Reference dentry (automount/reconfigure) or NULL
> + *
> + * Returns 0 on success or -ENOMEM on error.
> + */
> +static int smack_fs_context_init(struct fs_context *fc,
> +				 struct dentry *reference)
> +{
> +	struct superblock_smack *sbsp;
> +	struct smack_mnt_opts *ctx;
> +	struct inode_smack *isp;
> +
> +	ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
> +	if (!ctx)
> +		return -ENOMEM;
> +	fc->security = ctx;
> +
> +	if (fc->purpose == FS_CONTEXT_FOR_SUBMOUNT) {
> +		sbsp = smack_superblock(reference->d_sb);
> +		isp = smack_inode(reference->d_sb->s_root->d_inode);