NFSv4/pNFS: reject zero-length r_addr in nfs4_decode_mp_ds_addr

nfs4_decode_mp_ds_addr() decodes the r_netid and r_addr opaques of a
netaddr4 from a GETDEVICEINFO multipath-DS body, then immediately
calls strrchr(buf, '.') to locate the port separator. Both decodes
use xdr_stream_decode_string_dup(), and the current code checks only
"nlen < 0" / "rlen < 0" before dereferencing the returned string.

When the on-wire opaque has length zero, xdr_stream_decode_opaque_inline()
returns 0 and xdr_stream_decode_string_dup() falls through to its
"*str = NULL; return ret" tail, leaving buf NULL with a return value
of 0. The "< 0" check does not catch this, and the next line is
strrchr(NULL, '.'), a kernel NULL pointer dereference reachable from
any pNFS-flexfile client mounted against a malicious or compromised
metadata server.

Reject the zero-length cases explicitly so the decoder fails with
-EBADMSG (treated as a malformed GETDEVICEINFO body) instead of
panicking the client.

Cc: stable@vger.kernel.org
Fixes: 6b7f3cf963 ("nfs41: pull decode_ds_addr from file layout to generic pnfs")
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Signed-off-by: Anna Schumaker <anna.schumaker@hammerspace.com>

fs/nfs/pnfs_nfs.c

@@ -1075,14 +1075,14 @@ nfs4_decode_mp_ds_addr(struct net *net, struct xdr_stream *xdr, gfp_t gfp_flags)
 	/* r_netid */
 	nlen = xdr_stream_decode_string_dup(xdr, &netid, XDR_MAX_NETOBJ,
 					    gfp_flags);
-	if (unlikely(nlen < 0))
+	if (unlikely(nlen <= 0))
 		goto out_err;
 
 	/* r_addr: ip/ip6addr with port in dec octets - see RFC 5665 */
 	/* port is ".ABC.DEF", 8 chars max */
 	rlen = xdr_stream_decode_string_dup(xdr, &buf, INET6_ADDRSTRLEN +
 					    IPV6_SCOPE_ID_LEN + 8, gfp_flags);
-	if (unlikely(rlen < 0))
+	if (unlikely(rlen <= 0))
 		goto out_free_netid;
 
 	/* replace port '.' with '-' */