svcrpc: fix list-corrupting race on nfsd shutdown

After commit 3262c816a3d7fb1eaabce633caa317887ed549ae "[PATCH] knfsd:
split svc_serv into pools", svc_delete_xprt (then svc_delete_socket) no
longer removed its xpt_ready (then sk_ready) field from whatever list it
was on, noting that there was no point since the whole list was about to
be destroyed anyway.

That was mostly true, but forgot that a few svc_xprt_enqueue()'s might
still be hanging around playing with the about-to-be-destroyed list, and
could get themselves into trouble writing to freed memory if we left
this xprt on the list after freeing it.

(This is actually functionally identical to a patch made first by Ben
Greear, but with more comments.)

Cc: stable@kernel.org
Cc: gnb@fmeh.org
Reported-by: Ben Greear <greearb@candelatech.com>
Tested-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

1 files changed, 6 insertions, 5 deletions

diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
index ab86b7927f84..bd31208bbb61 100644
--- a/net/sunrpc/svc_xprt.c
+++ b/net/sunrpc/svc_xprt.c
@@ -902,12 +902,13 @@ void svc_delete_xprt(struct svc_xprt *xprt)
         if (!test_and_set_bit(XPT_DETACHED, &xprt->xpt_flags))
                 list_del_init(&xprt->xpt_list);
         /*
-         * We used to delete the transport from whichever list
-         * it's sk_xprt.xpt_ready node was on, but we don't actually
-         * need to.  This is because the only time we're called
-         * while still attached to a queue, the queue itself
-         * is about to be destroyed (in svc_destroy).
+         * The only time we're called while xpt_ready is still on a list
+         * is while the list itself is about to be destroyed (in
+         * svc_destroy).  BUT svc_xprt_enqueue could still be attempting
+         * to add new entries to the sp_sockets list, so we can't leave
+         * a freed xprt on it.
          */
+        list_del_init(&xprt->xpt_ready);
         if (test_bit(XPT_TEMP, &xprt->xpt_flags))
                 serv->sv_tmpcnt--;
         spin_unlock_bh(&serv->sv_lock);