[PATCH v3] nfsd: store net in svc_serv instead of deriving it from sv_permsocks

[Deepanshu Kartikey <kartikey406@gmail.com>]

nfsd() derives its struct net from the first entry of
serv->sv_permsocks using list_entry() without checking that the list
is non-empty. If the list is empty, list_entry() returns a bogus
svc_xprt computed from the list head embedded in struct svc_serv, and
the following perm_sock->xpt_net load reads past the end of the
svc_serv allocation:

  BUG: KASAN: slab-out-of-bounds in nfsd+0x118/0xbc0 fs/nfsd/nfssvc.c:894
  Read of size 8 at addr ffff88802d423f50 by task nfsd/5988

Rather than validating the list head before referencing it, store the
net in the svc_serv when it is created. svc_create_pooled() is only
used by nfsd, so add a struct net argument to it and record it in a
new sv_net field, then use that in nfsd().

Fixes: 88c476661719 ("nfsd: pass proper net to nfsd_destroy() from NFSd kthreads")
Reported-by: syzbot+561b859d032c8bf2fb53@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=561b859d032c8bf2fb53
Suggested-by: NeilBrown <neil@brown.name>
Tested-by: syzbot+561b859d032c8bf2fb53@syzkaller.appspotmail.com
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
v3: reword commit message, fix stray whitespace in svc_create_pooled()
    prototype (Neil)
v2: store the net in svc_serv via svc_create_pooled() instead of
    walking sv_permsocks under sv_lock (Jeff, Neil)
---
 fs/nfsd/nfssvc.c           | 5 ++---
 include/linux/sunrpc/svc.h | 2 ++
 net/sunrpc/svc.c           | 3 +++
 3 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
index 2edf716ea022..06a756d5636c 100644
--- a/fs/nfsd/nfssvc.c
+++ b/fs/nfsd/nfssvc.c
@@ -620,7 +620,7 @@ int nfsd_create_serv(struct net *net)
 		nfsd_max_blksize = nfsd_get_default_max_blksize();
 	nfsd_reset_versions(nn);
 	serv = svc_create_pooled(nfsd_programs, ARRAY_SIZE(nfsd_programs),
-				 &nn->nfsd_svcstats,
+				 &nn->nfsd_svcstats, net,
 				 nfsd_max_blksize, nfsd);
 	if (serv == NULL) {
 		percpu_ref_exit(&nn->nfsd_net_ref);
@@ -890,8 +890,7 @@ nfsd(void *vrqstp)
 {
 	struct svc_rqst *rqstp = (struct svc_rqst *) vrqstp;
 	struct svc_pool *pool = rqstp->rq_pool;
-	struct svc_xprt *perm_sock = list_entry(rqstp->rq_server->sv_permsocks.next, typeof(struct svc_xprt), xpt_list);
-	struct net *net = perm_sock->xpt_net;
+	struct net *net = rqstp->rq_server->sv_net;
 	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
 	struct nfsd_thread_local_info ntli = { };
 	bool have_mutex = false;
diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
index 2db1b9ec5658..49a5052b2670 100644
--- a/include/linux/sunrpc/svc.h
+++ b/include/linux/sunrpc/svc.h
@@ -72,6 +72,7 @@ enum {
 struct svc_serv {
 	struct svc_program *	sv_programs;	/* RPC programs */
 	struct svc_stat *	sv_stats;	/* RPC statistics */
+	struct net *sv_net;		/* only set for pooled (nfsd) services */
 	spinlock_t		sv_lock;
 	unsigned int		sv_nprogs;	/* Number of sv_programs */
 	unsigned int		sv_nrthreads;	/* # of running server threads */
@@ -461,6 +462,7 @@ void		   svc_exit_thread(struct svc_rqst *);
 struct svc_serv *  svc_create_pooled(struct svc_program *prog,
 				     unsigned int nprog,
 				     struct svc_stat *stats,
+				      struct net *net,
 				     unsigned int bufsize,
 				     int (*threadfn)(void *data));
 int		   svc_set_pool_threads(struct svc_serv *serv, struct svc_pool *pool,
diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
index 8297bad2b177..810c2db27fe3 100644
--- a/net/sunrpc/svc.c
+++ b/net/sunrpc/svc.c
@@ -481,6 +481,7 @@ EXPORT_SYMBOL_GPL(svc_create);
  * @prog:  Array of RPC programs the new service will handle
  * @nprogs: Number of programs in the array
  * @stats: the stats struct if desired
+ * @net: network namespace the service belongs to
  * @bufsize: maximum message size for @prog
  * @threadfn: a function to service RPC requests for @prog
  *
@@ -489,6 +490,7 @@ EXPORT_SYMBOL_GPL(svc_create);
 struct svc_serv *svc_create_pooled(struct svc_program *prog,
 				   unsigned int nprogs,
 				   struct svc_stat *stats,
+				   struct net *net,
 				   unsigned int bufsize,
 				   int (*threadfn)(void *data))
 {
@@ -502,6 +504,7 @@ struct svc_serv *svc_create_pooled(struct svc_program *prog,
 	if (!serv)
 		goto out_err;
 	serv->sv_is_pooled = true;
+	serv->sv_net = net;
 	return serv;
 out_err:
 	svc_pool_map_put();
-- 
2.43.0