control_if: Avoid heap-use-after-free in osmo_wqueue_bfd_cb

Imagine following scenario:
1- client connects to CTRL iface, a new conn is created with POLL_READ
enabled.
2- A non-related event happens which triggers a TRAP to be sent. As a
result, the wqueue for the conn has now enabled POLL_WRITE, and message
will be sent next time we go through osmo_main_select().
3- At the same time, we receive the GET cmd from the CTRL client, which
means POLL_READ event will be also triggered next time we call
osmo_main_select().
4- osmo_main_select triggers osmo_wqueue_bfd_cb with both READ/WRITE
flags set.
5- The read_cb of wqueue is executed first. The handler closes the CTRL
conn for some reason, freeing the osmo_fd struct and returns.
6- osmo_qeueue_bfd_cb keeps using the already freed osmo_fd and calls
write_cb.

So in step 6 we get a heap-use-after-free catched by AddressSanitizer:

20180424135406115 DLCTRL <0018> control_if.c:506 accept()ed new CTRL connection from (r=10.42.42.1:53910<->l=10.42.42.7:4249)
20180424135406116 DLCTRL <0018> control_cmd.c:378 Command: GET bts.0.oml-connection-state
20180424135406117 DLINP <0013> bts_ipaccess_nanobts.c:417 Identified BTS 1/0/0
20180424135406118 DNM <0005> abis_nm.c:1628 Get Attr (bts=0)
20180424135406118 DNM <0005> abis_nm.c:1628 Get Attr (bts=0)
20180424135406118 DCTRL <000e> osmo_bsc_ctrl.c:158 BTS connection (re)established, sending TRAP.
20180424135406119 DLCTRL <0018> control_if.c:173 close()d CTRL connection (r=10.42.42.1:53910<->l=10.42.42.7:4249)
=================================================================
==12301==ERROR: AddressSanitizer: heap-use-after-free on address 0x611000003e04 at pc 0x7f23091c3a2f bp 0x7ffc0cb73ff0 sp 0x7ffc0cb73fe8
READ of size 4 at 0x611000003e04 thread T0
    #0 0x7f23091c3a2e in osmo_wqueue_bfd_cb /home/osmocom-build/jenkins/workspace/osmo-gsm-tester_build-osmo-bsc/libosmocore/src/write_queue.c:65
    #1 0x7f23091ad5d8 in osmo_fd_disp_fds /home/osmocom-build/jenkins/workspace/osmo-gsm-tester_build-osmo-bsc/libosmocore/src/select.c:216
    #2 0x7f23091ad5d8 in osmo_select_main /home/osmocom-build/jenkins/workspace/osmo-gsm-tester_build-osmo-bsc/libosmocore/src/select.c:256
    #3 0x56538bdb7a26 in main /home/osmocom-build/jenkins/workspace/osmo-gsm-tester_build-osmo-bsc/osmo-bsc/src/osmo-bsc/osmo_bsc_main.c:532
    #4 0x7f23077532e0 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x202e0)
    #5 0x56538bdb8999 in _start (/home/jenkins/workspace/osmo-gsm-tester_run-prod/trial-896/inst/osmo-bsc/bin/osmo-bsc+0x259999)

Fixes: OS#3206

Change-Id: I84d10caaadcfa6bd46ba8756ca89aa0badcfd2e3

2 files changed, 25 insertions, 22 deletions

diff --git a/include/osmocom/core/write_queue.h b/include/osmocom/core/write_queue.h
index 2303f87e..071621d1 100644
--- a/include/osmocom/core/write_queue.h
+++ b/include/osmocom/core/write_queue.h
@@ -42,11 +42,11 @@ struct osmo_wqueue {
 	/*! actual linked list implementing the queue */
 	struct llist_head msg_queue;
 
-	/*! call-back in case qeueue is readable */
+	/*! call-back in case qeueue is readable. Return -EBADF if fd is freed inside cb. */
 	int (*read_cb)(struct osmo_fd *fd);
-	/*! call-back in case qeueue is writable */
+	/*! call-back in case qeueue is writable. Return -EBADF if fd is freed inside cb. */
 	int (*write_cb)(struct osmo_fd *fd, struct msgb *msg);
-	/*! call-back in case qeueue has exceptions */
+	/*! call-back in case qeueue has exceptions. Return -EBADF if fd is freed inside cb. */
 	int (*except_cb)(struct osmo_fd *fd);
 };
 
diff --git a/src/ctrl/control_if.c b/src/ctrl/control_if.c
index cc613ee7..0ba25125 100644
--- a/src/ctrl/control_if.c
+++ b/src/ctrl/control_if.c
@@ -327,7 +327,7 @@ err:
 
 static int handle_control_read(struct osmo_fd * bfd)
 {
-	int ret = -1;
+	int ret;
 	struct osmo_wqueue *queue;
 	struct ctrl_connection *ccon;
 	struct msgb *msg = NULL;
@@ -337,27 +337,28 @@ static int handle_control_read(struct osmo_fd * bfd)
 	ccon = container_of(queue, struct ctrl_connection, write_queue);
 
 	ret = ipa_msg_recv_buffered(bfd->fd, &msg, &ccon->pending_msg);
-	if (ret <= 0) {
-		if (ret == -EAGAIN)
-			/* received part of a message, it is stored in ccon->pending_msg and there's
-			 * nothing left to do now. */
-			return 0;
+	if (ret == 0) {
 		/* msg was already discarded. */
-		if (ret == 0) {
-			control_close_conn(ccon);
-			ret = -EIO;
-		}
-		else
-			LOGP(DLCTRL, LOGL_ERROR, "Failed to parse ip access message: %d\n", ret);
-
-		return ret;
+		goto close_fd;
+	} else if (ret == -EAGAIN) {
+		/* received part of a message, it is stored in ccon->pending_msg and there's
+		 * nothing left to do now. */
+		return 0;
+	} else if (ret < 0) {
+		LOGP(DLCTRL, LOGL_ERROR, "Failed to parse ip access message: %d\n", ret);
+		return 0;
 	}
 
 	ret = ctrl_handle_msg(ctrl, ccon, msg);
 	msgb_free(msg);
 	if (ret)
-		control_close_conn(ccon);
-	return ret;
+		goto close_fd;
+
+	return 0;
+
+close_fd:
+	control_close_conn(ccon);
+	return -EBADF;
 }
 
 int ctrl_handle_msg(struct ctrl_handle *ctrl, struct ctrl_connection *ccon, struct msgb *msg)
@@ -435,12 +436,14 @@ static int control_write_cb(struct osmo_fd *bfd, struct msgb *msg)
 	ccon = container_of(queue, struct ctrl_connection, write_queue);
 
 	rc = write(bfd->fd, msg->data, msg->len);
-	if (rc == 0)
+	if (rc == 0) {
 		control_close_conn(ccon);
-	else if (rc != msg->len)
+		return -EBADF;
+	}
+	if (rc != msg->len)
 		LOGP(DLCTRL, LOGL_ERROR, "Failed to write message to the CTRL connection.\n");
 
-	return rc;
+	return 0;
 }
 
 /*! Allocate CTRL connection