[3.19.y-ckt stable] Patch "cifs: fix race between call_async() and reconnect()" has been added to the 3.19.y-ckt tree

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    cifs: fix race between call_async() and reconnect()

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.19.y-queue

This patch is scheduled to be released in version 3.19.8-ckt14.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.19.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

---8<------------------------------------------------------------

>From 1513c88658fe253eb5cef5be6a2d9187cec5fb66 Mon Sep 17 00:00:00 2001
From: Rabin Vincent <rabin.vincent at axis.com>
Date: Wed, 23 Dec 2015 07:32:41 +0100
Subject: cifs: fix race between call_async() and reconnect()

commit 820962dc700598ffe8cd21b967e30e7520c34748 upstream.

cifs_call_async() queues the MID to the pending list and calls
smb_send_rqst().  If smb_send_rqst() performs a partial send, it sets
the tcpStatus to CifsNeedReconnect and returns an error code to
cifs_call_async().  In this case, cifs_call_async() removes the MID
from the list and returns to the caller.

However, cifs_call_async() releases the server mutex _before_ removing
the MID.  This means that a cifs_reconnect() can race with this function
and manage to remove the MID from the list and delete the entry before
cifs_call_async() calls cifs_delete_mid().  This leads to various
crashes due to the use after free in cifs_delete_mid().

Task1				Task2

cifs_call_async():
 - rc = -EAGAIN
 - mutex_unlock(srv_mutex)

				cifs_reconnect():
				 - mutex_lock(srv_mutex)
				 - mutex_unlock(srv_mutex)
				 - list_delete(mid)
				 - mid->callback()
				 	cifs_writev_callback():
				 		- mutex_lock(srv_mutex)
						- delete(mid)
				 		- mutex_unlock(srv_mutex)

 - cifs_delete_mid(mid) <---- use after free

Fix this by removing the MID in cifs_call_async() before releasing the
srv_mutex.  Also hold the srv_mutex in cifs_reconnect() until the MIDs
are moved out of the pending list.

Signed-off-by: Rabin Vincent <rabin.vincent at axis.com>
Acked-by: Shirish Pargaonkar <shirishpargaonkar at gmail.com>
Signed-off-by: Steve French <sfrench at localhost.localdomain>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 fs/cifs/connect.c   | 2 +-
 fs/cifs/transport.c | 6 ++++--
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/fs/cifs/connect.c b/fs/cifs/connect.c
index 2a772da..82ebe7d 100644
--- a/fs/cifs/connect.c
+++ b/fs/cifs/connect.c
@@ -357,7 +357,6 @@ cifs_reconnect(struct TCP_Server_Info *server)
 	server->session_key.response = NULL;
 	server->session_key.len = 0;
 	server->lstrp = jiffies;
-	mutex_unlock(&server->srv_mutex);

 	/* mark submitted MIDs for retry and issue callback */
 	INIT_LIST_HEAD(&retry_list);
@@ -370,6 +369,7 @@ cifs_reconnect(struct TCP_Server_Info *server)
 		list_move(&mid_entry->qhead, &retry_list);
 	}
 	spin_unlock(&GlobalMid_Lock);
+	mutex_unlock(&server->srv_mutex);

 	cifs_dbg(FYI, "%s: issuing mid callbacks\n", __func__);
 	list_for_each_safe(tmp, tmp2, &retry_list) {
diff --git a/fs/cifs/transport.c b/fs/cifs/transport.c
index 126f46b..66106f6 100644
--- a/fs/cifs/transport.c
+++ b/fs/cifs/transport.c
@@ -576,14 +576,16 @@ cifs_call_async(struct TCP_Server_Info *server, struct smb_rqst *rqst,
 	cifs_in_send_dec(server);
 	cifs_save_when_sent(mid);

-	if (rc < 0)
+	if (rc < 0) {
 		server->sequence_number -= 2;
+		cifs_delete_mid(mid);
+	}
+
 	mutex_unlock(&server->srv_mutex);

 	if (rc == 0)
 		return 0;

-	cifs_delete_mid(mid);
 	add_credits_and_wake_if(server, credits, optype);
 	return rc;
 }
--
1.9.1