[Qemu-devel] [RFC PATCH] char-socket: hold chr_write_lock during tcp_chr_free_connection()

[Alberto Garcia]

There's a race condition in which the tcp_chr_read() ioc handler can
close a connection that is being written to from another thread.

Running iotest 136 in a loop triggers this problem and crashes QEMU.

 (gdb) bt
 #0  0x00005558b842902d in object_get_class (obj=0x0) at qom/object.c:860
 #1  0x00005558b84f92db in qio_channel_writev_full (ioc=0x0, 
iov=0x7ffc355decf0, niov=1, fds=0x0, nfds=0, errp=0x0) at io/channel.c:76
 #2  0x00005558b84e0e9e in io_channel_send_full (ioc=0x0, buf=0x5558baf5beb0, 
len=138, fds=0x0, nfds=0) at chardev/char-io.c:123
 #3  0x00005558b84e4a69 in tcp_chr_write (chr=0x5558ba460380, 
buf=0x5558baf5beb0 "...", len=138) at chardev/char-socket.c:135
 #4  0x00005558b84dca55 in qemu_chr_write_buffer (s=0x5558ba460380, 
buf=0x5558baf5beb0 "...", len=138, offset=0x7ffc355dedd0, write_all=false) at 
chardev/char.c:112
 #5  0x00005558b84dcbc2 in qemu_chr_write (s=0x5558ba460380, buf=0x5558baf5beb0 
"...", len=138, write_all=false) at chardev/char.c:147
 #6  0x00005558b84dfb26 in qemu_chr_fe_write (be=0x5558ba476610, 
buf=0x5558baf5beb0 "...", len=138) at chardev/char-fe.c:42
 #7  0x00005558b8088c86 in monitor_flush_locked (mon=0x5558ba476610) at 
monitor.c:406
 #8  0x00005558b8088e8c in monitor_puts (mon=0x5558ba476610, str=0x5558ba921e49 
"") at monitor.c:449
 #9  0x00005558b8089178 in qmp_send_response (mon=0x5558ba476610, 
rsp=0x5558bb161600) at monitor.c:498
 #10 0x00005558b808920c in monitor_qapi_event_emit (event=QAPI_EVENT_SHUTDOWN, 
qdict=0x5558bb161600) at monitor.c:526
 #11 0x00005558b8089307 in monitor_qapi_event_queue_no_reenter 
(event=QAPI_EVENT_SHUTDOWN, qdict=0x5558bb161600) at monitor.c:551
 #12 0x00005558b80896c0 in qapi_event_emit (event=QAPI_EVENT_SHUTDOWN, 
qdict=0x5558bb161600) at monitor.c:626
 #13 0x00005558b855f23b in qapi_event_send_shutdown (guest=false, 
reason=SHUTDOWN_CAUSE_HOST_QMP_QUIT) at qapi/qapi-events-run-state.c:43
 #14 0x00005558b81911ef in qemu_system_shutdown 
(cause=SHUTDOWN_CAUSE_HOST_QMP_QUIT) at vl.c:1837
 #15 0x00005558b8191308 in main_loop_should_exit () at vl.c:1885
 #16 0x00005558b819140d in main_loop () at vl.c:1924
 #17 0x00005558b8198c84 in main (argc=18, argv=0x7ffc355df3f8, 
envp=0x7ffc355df490) at vl.c:4665

Signed-off-by: Alberto Garcia <address@hidden>
---
 chardev/char-socket.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/chardev/char-socket.c b/chardev/char-socket.c
index eaa8e8b68f..47bef1d2bd 100644
--- a/chardev/char-socket.c
+++ b/chardev/char-socket.c
@@ -148,7 +148,9 @@ static int tcp_chr_write(Chardev *chr, const uint8_t *buf, 
int len)
 
         if (ret < 0 && errno != EAGAIN) {
             if (tcp_chr_read_poll(chr) <= 0) {
+                qemu_mutex_unlock(&chr->chr_write_lock);
                 tcp_chr_disconnect(chr);
+                qemu_mutex_lock(&chr->chr_write_lock);
                 return len;
             } /* else let the read handler finish it properly */
         }
@@ -449,7 +451,9 @@ static void tcp_chr_disconnect(Chardev *chr)
     SocketChardev *s = SOCKET_CHARDEV(chr);
     bool emit_close = s->connected;
 
+    qemu_mutex_lock(&chr->chr_write_lock);
     tcp_chr_free_connection(chr);
+    qemu_mutex_unlock(&chr->chr_write_lock);
 
     if (s->listener) {
         qio_net_listener_set_client_func_full(s->listener, tcp_chr_accept,
-- 
2.11.0