[PATCH v2 4/4] ui/console: prevent use after free error

["Volker Rümelin" <vr_qemu@t-online.de>]

Make chr in the QemuConsole object a strong reference to the
referenced chardev device. This prevents a use after free error
if the chardev device goes away unexpectedly.

To reproduce the error start qemu-system built with address
sanitizer with the the following command line options.

-display sdl -chardev vc,id=test0,cols=132,rows=50.

Open the monitor console with CTRL-ALT-3 and remove the
unconnected chardev device test0.

(qemu) chardev-remove test0

Open the text console test0 with CTRL-ALT-2 and type a character.
QEMU immediately exits with this error message.

==28148==ERROR: AddressSanitizer: heap-use-after-free
  on address 0x60e000043778 at pc 0x558712ba7125
  bp 0x7fff270980b0 sp 0x7fff270980a8
READ of size 8 at 0x60e000043778 thread T0
    #0 0x558712ba7124 in qemu_chr_be_can_write
      ../qemu-master/chardev/char.c:188
    #1 0x558711624770 in kbd_send_chars
      ../qemu-master/ui/console.c:1113
    #2 0x558711634e91 in kbd_put_keysym_console
      ../qemu-master/ui/console.c:1175
    #3 0x55871163532a in kbd_put_string_console
      ../qemu-master/ui/console.c:1221
    #4 0x5587120a21e4 in handle_textinput
      ../qemu-master/ui/sdl2.c:464
    #5 0x5587120a21e4 in sdl2_poll_events
      ../qemu-master/ui/sdl2.c:650
    #6 0x5587116269c3 in dpy_refresh
      ../qemu-master/ui/console.c:1673
    #7 0x5587116269c3 in gui_update
      ../qemu-master/ui/console.c:158
    #8 0x558712d3a919 in timerlist_run_timers
      ../qemu-master/util/qemu-timer.c:573
    #9 0x558712d3b183 in qemu_clock_run_timers
      ../qemu-master/util/qemu-timer.c:587
    #10 0x558712d3b183 in qemu_clock_run_all_timers
      ../qemu-master/util/qemu-timer.c:669
    #11 0x558712d286d9 in main_loop_wait
      ../qemu-master/util/main-loop.c:542
    #12 0x5587123d313b in qemu_main_loop
      ../qemu-master/softmmu/runstate.c:726
    #13 0x5587115f989d in main
      ../qemu-master/softmmu/main.c:50
    #14 0x7f832ee0934c in __libc_start_main
      (/lib64/libc.so.6+0x2534c)
    #15 0x55871160b6e9 in _start
      (/home/ruemelin/rpmbuild/BUILD/qemu-6.1.50-build/
      qemu-system-x86_64+0x1f4f6e9)

Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
---
 ui/console.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/ui/console.c b/ui/console.c
index 29a3e3f0f5..1ef5a96295 100644
--- a/ui/console.c
+++ b/ui/console.c
@@ -2264,6 +2264,7 @@ static void vc_chr_open(Chardev *chr,
     }
 
     s->chr = chr;
+    object_ref(chr);
     drv->console = s;
 
     if (display_state) {
-- 
2.31.1