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Subject[PATCH 18/23] sunrpc: prevent use-after-free on clearing XPT_BUSY review patch.  If anyone has any objections, please let us know.


From: NeilBrown <>

commit ed2849d3ecfa339435818eeff28f6c3424300cec upstream.

When an xprt is created, it has a refcount of 1, and XPT_BUSY is set.
The refcount is *not* owned by the thread that created the xprt
(as is clear from the fact that creators never put the reference).
Rather, it is owned by the absence of XPT_DEAD. Once XPT_DEAD is set,
(And XPT_BUSY is clear) that initial reference is dropped and the xprt
can be freed.

So when a creator clears XPT_BUSY it is dropping its only reference and
so must not touch the xprt again.

However svc_recv, after calling ->xpo_accept (and so getting an XPT_BUSY
reference on a new xprt), calls svc_xprt_recieved. This clears
XPT_BUSY and then svc_xprt_enqueue - this last without owning a reference.
This is dangerous and has been seen to leave svc_xprt_enqueue working
with an xprt containing garbage.

So we need to hold an extra counted reference over that call to

For safety, any time we clear XPT_BUSY and then use the xprt again, we
first get a reference, and the put it again afterwards.

Note that svc_close_all does not need this extra protection as there are
no threads running, and the final free can only be called asynchronously
from such a thread.

Signed-off-by: NeilBrown <>
Signed-off-by: J. Bruce Fields <>
Signed-off-by: Greg Kroah-Hartman <>
Signed-off-by: Willy Tarreau <>

net/sunrpc/svc_xprt.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

Index: longterm-2.6.27/net/sunrpc/svc_xprt.c
--- longterm-2.6.27.orig/net/sunrpc/svc_xprt.c 2011-01-29 11:19:14.731064813 +0100
+++ longterm-2.6.27/net/sunrpc/svc_xprt.c 2011-01-29 11:27:22.220064064 +0100
@@ -172,6 +172,7 @@
list_for_each_entry(xcl, &svc_xprt_class_list, xcl_list) {
struct svc_xprt *newxprt;
+ unsigned short newport;

if (strcmp(xprt_name, xcl->xcl_name))
@@ -192,8 +193,9 @@
list_add(&newxprt->xpt_list, &serv->sv_permsocks);
+ newport = svc_xprt_local_port(newxprt);
clear_bit(XPT_BUSY, &newxprt->xpt_flags);
- return svc_xprt_local_port(newxprt);
+ return newport;
@@ -386,8 +388,13 @@
BUG_ON(!test_bit(XPT_BUSY, &xprt->xpt_flags));
xprt->xpt_pool = NULL;
+ /* As soon as we clear busy, the xprt could be closed and
+ * 'put', so we need a reference to call svc_xprt_enqueue with:
+ */
+ svc_xprt_get(xprt);
clear_bit(XPT_BUSY, &xprt->xpt_flags);
+ svc_xprt_put(xprt);

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