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Subject[PATCH 3.2 51/87] cifs: fix race between call_async() and reconnect()
3.2.77-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Rabin Vincent <rabin.vincent@axis.com>

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@axis.com>
Acked-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Signed-off-by: Steve French <sfrench@localhost.localdomain>
[bwh: Backported to 3.2:
- In cifs_call_async() there are two error paths jumping to 'out_err';
fix both of them
- s/cifs_delete_mid/delete_mid/
- Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
--- a/fs/cifs/connect.c
+++ b/fs/cifs/connect.c
@@ -134,7 +134,6 @@ cifs_reconnect(struct TCP_Server_Info *s
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);
@@ -147,6 +146,7 @@ cifs_reconnect(struct TCP_Server_Info *s
list_move(&mid_entry->qhead, &retry_list);
}
spin_unlock(&GlobalMid_Lock);
+ mutex_unlock(&server->srv_mutex);

cFYI(1, "%s: issuing mid callbacks", __func__);
list_for_each_safe(tmp, tmp2, &retry_list) {
--- a/fs/cifs/transport.c
+++ b/fs/cifs/transport.c
@@ -370,10 +370,8 @@ cifs_call_async(struct TCP_Server_Info *
spin_unlock(&GlobalMid_Lock);

rc = cifs_sign_smb2(iov, nvec, server, &mid->sequence_number);
- if (rc) {
- mutex_unlock(&server->srv_mutex);
- goto out_err;
- }
+ if (rc)
+ goto out;

mid->receive = receive;
mid->callback = callback;
@@ -384,14 +382,15 @@ cifs_call_async(struct TCP_Server_Info *
rc = smb_sendv(server, iov, nvec);
cifs_in_send_dec(server);
cifs_save_when_sent(mid);
+out:
+ if (rc < 0)
+ delete_mid(mid);
+
mutex_unlock(&server->srv_mutex);

- if (rc)
- goto out_err;
+ if (rc == 0)
+ return 0;

- return rc;
-out_err:
- delete_mid(mid);
atomic_dec(&server->inFlight);
wake_up(&server->request_q);
return rc;
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