From: Vasily Averin <vvs@virtuozzo.com>
Date: Mon, 24 Dec 2018 14:44:52 +0300
Subject: [PATCH] sunrpc: use-after-free in svc_process_common()
Git-commit: d4b09acf924b84bae77cad090a9d108e70b43643
Patch-mainline: v5.0-rc1
References: bsc#1119946 CVE-2018-16884

if node have NFSv41+ mounts inside several net namespaces
it can lead to use-after-free in svc_process_common()

svc_process_common()
        /* Setup reply header */
        rqstp->rq_xprt->xpt_ops->xpo_prep_reply_hdr(rqstp); <<< HERE

svc_process_common() can use incorrect rqstp->rq_xprt,
its caller function bc_svc_process() takes it from serv->sv_bc_xprt.
The problem is that serv is global structure but sv_bc_xprt
is assigned per-netnamespace.

According to Trond, the whole "let's set up rqstp->rq_xprt
for the back channel" is nothing but a giant hack in order
to work around the fact that svc_process_common() uses it
to find the xpt_ops, and perform a couple of (meaningless
for the back channel) tests of xpt_flags.

All we really need in svc_process_common() is to be able to run
rqstp->rq_xprt->xpt_ops->xpo_prep_reply_hdr()

Bruce J Fields points that this xpo_prep_reply_hdr() call
is an awfully roundabout way just to do "svc_putnl(resv, 0);"
in the tcp case.

This patch does not initialiuze rqstp->rq_xprt in bc_svc_process(),
now it calls svc_process_common() with rqstp->rq_xprt = NULL.

To adjust reply header svc_process_common() just check
rqstp->rq_prot and calls svc_tcp_prep_reply_hdr() for tcp case.

To handle rqstp->rq_xprt = NULL case in functions called from
svc_process_common() patch intruduces net namespace pointer
svc_rqst->rq_bc_net and adjust SVC_NET() definition.
Some other function was also adopted to properly handle described case.

Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: NeilBrown <neilb@suse.com>
Acked-by: NeilBrown <neilb@suse.com>

---
 include/linux/sunrpc/svc.h |    5 ++++-
 net/sunrpc/svc.c           |   13 +++++++++----
 net/sunrpc/svc_xprt.c      |    5 +++--
 3 files changed, 16 insertions(+), 7 deletions(-)

--- a/include/linux/sunrpc/svc.h
+++ b/include/linux/sunrpc/svc.h
@@ -291,9 +291,12 @@ struct svc_rqst {
 	struct svc_cacherep *	rq_cacherep;	/* cache info */
 	struct task_struct	*rq_task;	/* service thread */
 	spinlock_t		rq_lock;	/* per-request lock */
+	struct net		*rq_bc_net;	/* pointer to backchannel's
+						 * net namespace
+						 */
 };
 
-#define SVC_NET(svc_rqst)	(svc_rqst->rq_xprt->xpt_net)
+#define SVC_NET(rqst) (rqst->rq_xprt ? rqst->rq_xprt->xpt_net : rqst->rq_bc_net)
 
 /*
  * Rigorous type checking on sockaddr type conversions
--- a/net/sunrpc/svc.c
+++ b/net/sunrpc/svc.c
@@ -1173,7 +1173,12 @@ svc_process_common(struct svc_rqst *rqst
 	clear_bit(RQ_DROPME, &rqstp->rq_flags);
 
 	/* Setup reply header */
-	rqstp->rq_xprt->xpt_ops->xpo_prep_reply_hdr(rqstp);
+	if (rqstp->rq_prot == IPPROTO_TCP) {
+		struct kvec *resv = &rqstp->rq_res.head[0];
+
+		/* tcp needs a space for the record length... */
+		svc_putnl(resv, 0);
+	}
 
 	svc_putu32(resv, rqstp->rq_xid);
 
@@ -1245,7 +1250,7 @@ svc_process_common(struct svc_rqst *rqst
 	 * for lower versions. RPC_PROG_MISMATCH seems to be the closest
 	 * fit.
 	 */
-	if (versp->vs_need_cong_ctrl &&
+	if (versp->vs_need_cong_ctrl && rqstp->rq_xprt &&
 	    !test_bit(XPT_CONG_CTRL, &rqstp->rq_xprt->xpt_flags))
 		goto err_bad_vers;
 
@@ -1334,7 +1339,7 @@ svc_process_common(struct svc_rqst *rqst
 	return 0;
 
  close:
-	if (test_bit(XPT_TEMP, &rqstp->rq_xprt->xpt_flags))
+	if (rqstp->rq_xprt && test_bit(XPT_TEMP, &rqstp->rq_xprt->xpt_flags))
 		svc_close_xprt(rqstp->rq_xprt);
 	dprintk("svc: svc_process close\n");
 	return 0;
@@ -1461,10 +1466,10 @@ bc_svc_process(struct svc_serv *serv, st
 	dprintk("svc: %s(%p)\n", __func__, req);
 
 	/* Build the svc_rqst used by the common processing routine */
-	rqstp->rq_xprt = serv->sv_bc_xprt;
 	rqstp->rq_xid = req->rq_xid;
 	rqstp->rq_prot = req->rq_xprt->prot;
 	rqstp->rq_server = serv;
+	rqstp->rq_bc_net = req->rq_xprt->xprt_net;
 
 	rqstp->rq_addrlen = sizeof(req->rq_xprt->addr);
 	memcpy(&rqstp->rq_addr, &req->rq_xprt->addr, rqstp->rq_addrlen);
--- a/net/sunrpc/svc_xprt.c
+++ b/net/sunrpc/svc_xprt.c
@@ -510,10 +510,11 @@ out:
  */
 void svc_reserve(struct svc_rqst *rqstp, int space)
 {
+	struct svc_xprt *xprt = rqstp->rq_xprt;
+
 	space += rqstp->rq_res.head[0].iov_len;
 
-	if (space < rqstp->rq_reserved) {
-		struct svc_xprt *xprt = rqstp->rq_xprt;
+	if (xprt && space < rqstp->rq_reserved) {
 		atomic_sub((rqstp->rq_reserved - space), &xprt->xpt_reserved);
 		rqstp->rq_reserved = space;