nfsd4: fix unlikely race in session replay case

In the replay case, the

	renew_client(session->se_client);

happens after we've droppped the sessionid_lock, and without holding a
reference on the session; so there's nothing preventing the session
being freed before we get here.

Thanks to Benny Halevy for catching a bug in an earlier version of this
patch.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Acked-by: Benny Halevy <bhalevy@panasas.com>

3 files changed, 10 insertions, 8 deletions

diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
index e147dbcb7ef7..61282f8405b5 100644
--- a/fs/nfsd/nfs4proc.c
+++ b/fs/nfsd/nfs4proc.c
@@ -1027,6 +1027,7 @@ nfsd4_proc_compound(struct svc_rqst *rqstp,
         resp->rqstp = rqstp;
         resp->cstate.minorversion = args->minorversion;
         resp->cstate.replay_owner = NULL;
+        resp->cstate.session = NULL;
         fh_init(&resp->cstate.current_fh, NFS4_FHSIZE);
         fh_init(&resp->cstate.save_fh, NFS4_FHSIZE);
         /* Use the deferral mechanism only for NFSv4.0 compounds */
diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index bba9fff49cfe..737315c61e7e 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -1443,11 +1443,10 @@ nfsd4_sequence(struct svc_rqst *rqstp,
         cstate->slot = slot;
         cstate->session = session;
 
-        /* Hold a session reference until done processing the compound:
-         * nfsd4_put_session called only if the cstate slot is set.
-         */
-        nfsd4_get_session(session);
 out:
+        /* Hold a session reference until done processing the compound. */
+        if (cstate->session)
+                nfsd4_get_session(cstate->session);
         spin_unlock(&sessionid_lock);
         /* Renew the clientid on success and on replay */
         if (cstate->session) {
diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
index fb27b1db007b..05bc5bd63c95 100644
--- a/fs/nfsd/nfs4xdr.c
+++ b/fs/nfsd/nfs4xdr.c
@@ -3306,10 +3306,12 @@ nfs4svc_encode_compoundres(struct svc_rqst *rqstp, __be32 *p, struct nfsd4_compo
                 iov = &rqstp->rq_res.head[0];
         iov->iov_len = ((char*)resp->p) - (char*)iov->iov_base;
         BUG_ON(iov->iov_len > PAGE_SIZE);
-        if (nfsd4_has_session(cs) && cs->status != nfserr_replay_cache) {
-                nfsd4_store_cache_entry(resp);
-                dprintk("%s: SET SLOT STATE TO AVAILABLE\n", __func__);
-                resp->cstate.slot->sl_inuse = false;
+        if (nfsd4_has_session(cs)) {
+                if (cs->status != nfserr_replay_cache) {
+                        nfsd4_store_cache_entry(resp);
+                        dprintk("%s: SET SLOT STATE TO AVAILABLE\n", __func__);
+                        resp->cstate.slot->sl_inuse = false;
+                }
                 nfsd4_put_session(resp->cstate.session);
         }
         return 1;