SUNRPC: Fix GSS privacy computation of auth->au_ralign

When the au_ralign field was added to gss_unwrap_resp_priv, the
wrong calculation was used. Setting au_rslack == au_ralign is
probably correct for kerberos_v1 privacy, but kerberos_v2 privacy
adds additional GSS data after the clear text RPC message.
au_ralign needs to be smaller than au_rslack in that fairly common
case.

When xdr_buf_trim() is restored to gss_unwrap_kerberos_v2(), it does
exactly what I feared it would: it trims off part of the clear text
RPC message. However, that's because rpc_prepare_reply_pages() does
not set up the rq_rcv_buf's tail correctly because au_ralign is too
large.

Fixing the au_ralign computation also corrects the alignment of
rq_rcv_buf->pages so that the client does not have to shift reply
data payloads after they are received.

Fixes: 35e77d21baa0 ("SUNRPC: Add rpc_auth::au_ralign field")
Signed-off-by: Chuck Lever <[e-mail]>

include/linux/sunrpc/gss_api.h

···
 struct gss_ctx {
 	struct gss_api_mech	*mech_type;
 	void			*internal_ctx_id;
+	unsigned int		slack, align;
 };
 
 #define GSS_C_NO_BUFFER		((struct xdr_netobj) 0)

net/sunrpc/auth_gss/auth_gss.c

···
 	struct xdr_buf *rcv_buf = &rqstp->rq_rcv_buf;
 	struct kvec *head = rqstp->rq_rcv_buf.head;
 	struct rpc_auth *auth = cred->cr_auth;
-	unsigned int savedlen = rcv_buf->len;
 	u32 offset, opaque_len, maj_stat;
 	__be32 *p;
 
···
 	 */
 	xdr_init_decode(xdr, rcv_buf, p, rqstp);
 
-	auth->au_rslack = auth->au_verfsize + 2 +
-			  XDR_QUADLEN(savedlen - rcv_buf->len);
-	auth->au_ralign = auth->au_verfsize + 2 +
-			  XDR_QUADLEN(savedlen - rcv_buf->len);
+	auth->au_rslack = auth->au_verfsize + 2 + ctx->gc_gss_ctx->slack;
+	auth->au_ralign = auth->au_verfsize + 2 + ctx->gc_gss_ctx->align;
+
 	return 0;
 unwrap_failed:
 	trace_rpcgss_unwrap_failed(task);

net/sunrpc/auth_gss/gss_krb5_wrap.c

···
 
 static u32
 gss_unwrap_kerberos_v1(struct krb5_ctx *kctx, int offset, int len,
-		       struct xdr_buf *buf)
+		       struct xdr_buf *buf, unsigned int *slack,
+		       unsigned int *align)
 {
 	int			signalg;
 	int			sealalg;
···
 	u32			conflen = kctx->gk5e->conflen;
 	int			crypt_offset;
 	u8			*cksumkey;
+	unsigned int		saved_len = buf->len;
 
 	dprintk("RPC:       gss_unwrap_kerberos\n");
 
···
 	if (gss_krb5_remove_padding(buf, blocksize))
 		return GSS_S_DEFECTIVE_TOKEN;
 
+	/* slack must include room for krb5 padding */
+	*slack = XDR_QUADLEN(saved_len - buf->len);
+	/* The GSS blob always precedes the RPC message payload */
+	*align = *slack;
 	return GSS_S_COMPLETE;
 }
 
···
 
 static u32
 gss_unwrap_kerberos_v2(struct krb5_ctx *kctx, int offset, int len,
-		       struct xdr_buf *buf)
+		       struct xdr_buf *buf, unsigned int *slack,
+		       unsigned int *align)
 {
 	time64_t	now;
 	u8		*ptr;
···
 	/* Trim off the trailing "extra count" and checksum blob */
 	buf->len -= ec + GSS_KRB5_TOK_HDR_LEN + tailskip;
 
+	*align = XDR_QUADLEN(GSS_KRB5_TOK_HDR_LEN + headskip);
+	*slack = *align + XDR_QUADLEN(ec + GSS_KRB5_TOK_HDR_LEN + tailskip);
 	return GSS_S_COMPLETE;
 }
 
···
 	case ENCTYPE_DES_CBC_RAW:
 	case ENCTYPE_DES3_CBC_RAW:
 	case ENCTYPE_ARCFOUR_HMAC:
-		return gss_unwrap_kerberos_v1(kctx, offset, len, buf);
+		return gss_unwrap_kerberos_v1(kctx, offset, len, buf,
+					      &gctx->slack, &gctx->align);
 	case ENCTYPE_AES128_CTS_HMAC_SHA1_96:
 	case ENCTYPE_AES256_CTS_HMAC_SHA1_96:
-		return gss_unwrap_kerberos_v2(kctx, offset, len, buf);
+		return gss_unwrap_kerberos_v2(kctx, offset, len, buf,
+					      &gctx->slack, &gctx->align);
 	}
 }