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Subject[068/129] svcrpc: avoid memory-corruption on pool shutdown
3.2-stable review patch.  If anyone has any objections, please let me know.

------------------
Content-Length: 5240
Lines: 148
From: "J. Bruce Fields" <bfields@redhat.com>

commit b4f36f88b3ee7cf26bf0be84e6c7fc15f84dcb71 upstream.

Socket callbacks use svc_xprt_enqueue() to add an xprt to a
pool->sp_sockets list. In normal operation a server thread will later
come along and take the xprt off that list. On shutdown, after all the
threads have exited, we instead manually walk the sv_tempsocks and
sv_permsocks lists to find all the xprt's and delete them.

So the sp_sockets lists don't really matter any more. As a result,
we've mostly just ignored them and hoped they would go away.

Which has gotten us into trouble; witness for example ebc63e531cc6
"svcrpc: fix list-corrupting race on nfsd shutdown", the result of Ben
Greear noticing that a still-running svc_xprt_enqueue() could re-add an
xprt to an sp_sockets list just before it was deleted. The fix was to
remove it from the list at the end of svc_delete_xprt(). But that only
made corruption less likely--I can see nothing that prevents a
svc_xprt_enqueue() from adding another xprt to the list at the same
moment that we're removing this xprt from the list. In fact, despite
the earlier xpo_detach(), I don't even see what guarantees that
svc_xprt_enqueue() couldn't still be running on this xprt.

So, instead, note that svc_xprt_enqueue() essentially does:
lock sp_lock
if XPT_BUSY unset
add to sp_sockets
unlock sp_lock

So, if we do:

set XPT_BUSY on every xprt.
Empty every sp_sockets list, under the sp_socks locks.

Then we're left knowing that the sp_sockets lists are all empty and will
stay that way, since any svc_xprt_enqueue() will check XPT_BUSY under
the sp_lock and see it set.

And *then* we can continue deleting the xprt's.

(Thanks to Jeff Layton for being correctly suspicious of this code....)
Cc: Ben Greear <greearb@candelatech.com>
Cc: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
net/sunrpc/svc.c | 10 +++++++++-
net/sunrpc/svc_xprt.c | 48 +++++++++++++++++++++++++++++-------------------
2 files changed, 38 insertions(+), 20 deletions(-)
--- a/net/sunrpc/svc.c
+++ b/net/sunrpc/svc.c
@@ -530,7 +530,15 @@ svc_destroy(struct svc_serv *serv)
printk("svc_destroy: no threads for serv=%p!\n", serv);

del_timer_sync(&serv->sv_temptimer);
-
+ /*
+ * The set of xprts (contained in the sv_tempsocks and
+ * sv_permsocks lists) is now constant, since it is modified
+ * only by accepting new sockets (done by service threads in
+ * svc_recv) or aging old ones (done by sv_temptimer), or
+ * configuration changes (excluded by whatever locking the
+ * caller is using--nfsd_mutex in the case of nfsd). So it's
+ * safe to traverse those lists and shut everything down:
+ */
svc_close_all(serv);

if (serv->sv_shutdown)
--- a/net/sunrpc/svc_xprt.c
+++ b/net/sunrpc/svc_xprt.c
@@ -893,14 +893,7 @@ void svc_delete_xprt(struct svc_xprt *xp
spin_lock_bh(&serv->sv_lock);
if (!test_and_set_bit(XPT_DETACHED, &xprt->xpt_flags))
list_del_init(&xprt->xpt_list);
- /*
- * 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);
+ BUG_ON(!list_empty(&xprt->xpt_ready));
if (test_bit(XPT_TEMP, &xprt->xpt_flags))
serv->sv_tmpcnt--;
spin_unlock_bh(&serv->sv_lock);
@@ -931,28 +924,45 @@ EXPORT_SYMBOL_GPL(svc_close_xprt);
static void svc_close_list(struct list_head *xprt_list)
{
struct svc_xprt *xprt;
- struct svc_xprt *tmp;

- /*
- * The server is shutting down, and no more threads are running.
- * svc_xprt_enqueue() might still be running, but at worst it
- * will re-add the xprt to sp_sockets, which will soon get
- * freed. So we don't bother with any more locking, and don't
- * leave the close to the (nonexistent) server threads:
- */
- list_for_each_entry_safe(xprt, tmp, xprt_list, xpt_list) {
+ list_for_each_entry(xprt, xprt_list, xpt_list) {
set_bit(XPT_CLOSE, &xprt->xpt_flags);
- svc_delete_xprt(xprt);
+ set_bit(XPT_BUSY, &xprt->xpt_flags);
}
}

void svc_close_all(struct svc_serv *serv)
{
+ struct svc_pool *pool;
+ struct svc_xprt *xprt;
+ struct svc_xprt *tmp;
+ int i;
+
svc_close_list(&serv->sv_tempsocks);
svc_close_list(&serv->sv_permsocks);
+
+ for (i = 0; i < serv->sv_nrpools; i++) {
+ pool = &serv->sv_pools[i];
+
+ spin_lock_bh(&pool->sp_lock);
+ while (!list_empty(&pool->sp_sockets)) {
+ xprt = list_first_entry(&pool->sp_sockets, struct svc_xprt, xpt_ready);
+ list_del_init(&xprt->xpt_ready);
+ }
+ spin_unlock_bh(&pool->sp_lock);
+ }
+ /*
+ * At this point the sp_sockets lists will stay empty, since
+ * svc_enqueue will not add new entries without taking the
+ * sp_lock and checking XPT_BUSY.
+ */
+ list_for_each_entry_safe(xprt, tmp, &serv->sv_tempsocks, xpt_list)
+ svc_delete_xprt(xprt);
+ list_for_each_entry_safe(xprt, tmp, &serv->sv_permsocks, xpt_list)
+ svc_delete_xprt(xprt);
+
BUG_ON(!list_empty(&serv->sv_permsocks));
BUG_ON(!list_empty(&serv->sv_tempsocks));
-
}

/*



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