linux-kernel - [36/50] svcrpc: fix list-corrupting race on nfsd shutdown

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Date:	Fri, 05 Aug 2011 17:03:54 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk, gnb@...h.org,
	"J. Bruce Fields" <bfields@...hat.com>
Subject: [36/50] svcrpc: fix list-corrupting race on nfsd shutdown

2.6.33-longterm review patch.  If anyone has any objections, please let us know.

------------------

From: "J. Bruce Fields" <bfields@...hat.com>

commit ebc63e531cc6a457595dd110b07ac530eae788c3 upstream.

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: gnb@...h.org
Reported-by: Ben Greear <greearb@...delatech.com>
Tested-by: Ben Greear <greearb@...delatech.com>
Signed-off-by: J. Bruce Fields <bfields@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 net/sunrpc/svc_xprt.c |   11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

--- a/net/sunrpc/svc_xprt.c
+++ b/net/sunrpc/svc_xprt.c
@@ -884,12 +884,13 @@ void svc_delete_xprt(struct svc_xprt *xp
 	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--;

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