cifs: Fix use-after-free on mid_q_entry

With CIFS_DEBUG_2 enabled, additional debug information is tracked inside each
mid_q_entry struct, however cifs_save_when_sent may use the mid_q_entry after it
has been freed from the appropriate callback if the transport layer has very low
latency. Holding the srv_mutex fixes this use-after-free, as cifs_save_when_sent
is called while the srv_mutex is held while the request is sent.
Signed-off-by: Christopher Oo <t-chriso@microsoft.com>

fs/cifs/cifssmb.c

@@ -696,7 +696,9 @@ cifs_echo_callback(struct mid_q_entry *mid)
 {
 	struct TCP_Server_Info *server = mid->callback_data;
 
+	mutex_lock(&server->srv_mutex);
 	DeleteMidQEntry(mid);
+	mutex_unlock(&server->srv_mutex);
 	add_credits(server, 1, CIFS_ECHO_OP);
 }
 
@@ -1572,7 +1574,9 @@ cifs_readv_callback(struct mid_q_entry *mid)
 	}
 
 	queue_work(cifsiod_wq, &rdata->work);
+	mutex_lock(&server->srv_mutex);
 	DeleteMidQEntry(mid);
+	mutex_unlock(&server->srv_mutex);
 	add_credits(server, 1, 0);
 }
 
@@ -2032,6 +2036,7 @@ cifs_writev_callback(struct mid_q_entry *mid)
 {
 	struct cifs_writedata *wdata = mid->callback_data;
 	struct cifs_tcon *tcon = tlink_tcon(wdata->cfile->tlink);
+	struct TCP_Server_Info *server = tcon->ses->server;
 	unsigned int written;
 	WRITE_RSP *smb = (WRITE_RSP *)mid->resp_buf;
 
@@ -2068,7 +2073,9 @@ cifs_writev_callback(struct mid_q_entry *mid)
 	}
 
 	queue_work(cifsiod_wq, &wdata->work);
+	mutex_lock(&server->srv_mutex);
 	DeleteMidQEntry(mid);
+	mutex_unlock(&server->srv_mutex);
 	add_credits(tcon->ses->server, 1, 0);
 }
 

fs/cifs/smb2pdu.c

@@ -1626,7 +1626,9 @@ smb2_echo_callback(struct mid_q_entry *mid)
 	if (mid->mid_state == MID_RESPONSE_RECEIVED)
 		credits_received = le16_to_cpu(smb2->hdr.CreditRequest);
 
+	mutex_lock(&server->srv_mutex);
 	DeleteMidQEntry(mid);
+	mutex_unlock(&server->srv_mutex);
 	add_credits(server, credits_received, CIFS_ECHO_OP);
 }
 
@@ -1810,7 +1812,9 @@ smb2_readv_callback(struct mid_q_entry *mid)
 		cifs_stats_fail_inc(tcon, SMB2_READ_HE);
 
 	queue_work(cifsiod_wq, &rdata->work);
+	mutex_lock(&server->srv_mutex);
 	DeleteMidQEntry(mid);
+	mutex_unlock(&server->srv_mutex);
 	add_credits(server, credits_received, 0);
 }
 
@@ -1938,6 +1942,7 @@ smb2_writev_callback(struct mid_q_entry *mid)
 {
 	struct cifs_writedata *wdata = mid->callback_data;
 	struct cifs_tcon *tcon = tlink_tcon(wdata->cfile->tlink);
+	struct TCP_Server_Info *server = tcon->ses->server;
 	unsigned int written;
 	struct smb2_write_rsp *rsp = (struct smb2_write_rsp *)mid->resp_buf;
 	unsigned int credits_received = 1;
@@ -1977,7 +1982,9 @@ smb2_writev_callback(struct mid_q_entry *mid)
 		cifs_stats_fail_inc(tcon, SMB2_WRITE_HE);
 
 	queue_work(cifsiod_wq, &wdata->work);
+	mutex_lock(&server->srv_mutex);
 	DeleteMidQEntry(mid);
+	mutex_unlock(&server->srv_mutex);
 	add_credits(tcon->ses->server, credits_received, 0);
 }
 

fs/cifs/transport.c

@@ -644,7 +644,9 @@ cifs_sync_mid_result(struct mid_q_entry *mid, struct TCP_Server_Info *server)
 	}
 	spin_unlock(&GlobalMid_Lock);
 
+	mutex_lock(&server->srv_mutex);
 	DeleteMidQEntry(mid);
+	mutex_unlock(&server->srv_mutex);
 	return rc;
 }