Re: [PATCH v3 3/3] NFSD: Sign filehandles

[Eric Biggers <ebiggers@kernel.org>]

On Mon, Feb 02, 2026 at 11:19:38AM -0500, Benjamin Coddington wrote:
> +/*
> + * Append an 8-byte MAC to the filehandle hashed from the server's fh_key:
> + */
> +static int fh_append_mac(struct svc_fh *fhp, struct net *net)
> +{
> +	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
> +	struct knfsd_fh *fh = &fhp->fh_handle;
> +	siphash_key_t *fh_key = nn->fh_key;
> +	u64 hash;
> +
> +	if (!(fhp->fh_export->ex_flags & NFSEXP_SIGN_FH))
> +		return 0;
> +
> +	if (!fh_key) {
> +		pr_warn_ratelimited("NFSD: unable to sign filehandles, fh_key not set.\n");
> +		return -EINVAL;
> +	}
> +
> +	if (fh->fh_size + sizeof(hash) > fhp->fh_maxsize) {
> +		pr_warn_ratelimited("NFSD: unable to sign filehandles, fh_size %d would be greater"
> +			" than fh_maxsize %d.\n", (int)(fh->fh_size + sizeof(hash)), fhp->fh_maxsize);
> +		return -EINVAL;
> +	}
> +
> +	hash = siphash(&fh->fh_raw, fh->fh_size, fh_key);
> +	memcpy(&fh->fh_raw[fh->fh_size], &hash, sizeof(hash));
> +	fh->fh_size += sizeof(hash);
> +
> +	return 0;

Note that this is still creating endianness-specific MAC values,
considering that siphash() returns a u64 which is being copied directly
into a byte array.  Maybe not what was intended?

- Eric