[Qemu-devel] [Bug 1784900] Re: QEMU (frontend) crashes upon warm reboot with virtio-gpu device and vga=775 on Linux cmdline

[Stefan Berger <1784900@bugs.launchpad.net>]

The reason for this bug is memory corruption in glibc's memory chunk
header that is in front of some bitmap pixman is allocating and
maintaining as image->bits.free_me. I set a memory watchpoint to this
memory location and this code here triggered it and corrupted what seems
to be a memory chunk size indicator, which upon free() causes print of
'invalid pointer' by glibc:

Thread 1 "qemu-system-x86" hit Hardware watchpoint 2: *0x7f6160361d88

Old value = 3145749
New value = 0
vga_draw_line8 (vga=vga@entry=0x556d68549b30, d=0x7f6160361d80 "", d@entry=0x7f61603615e0 "", addr=983528, width=<optimized out>)
    at /home/stefanb/tmp/qemu-tip/hw/display/vga-helpers.h:297
297	        ((uint32_t *)d)[3] = palette[vga_read_byte(vga, addr + 3)];


(gdb) bt
#0  vga_draw_line8 (vga=vga@entry=0x556d68549b30, d=0x7f6160361d80 "", d@entry=0x7f61603615e0 "", addr=983528, width=<optimized out>)
    at /home/stefanb/tmp/qemu-tip/hw/display/vga-helpers.h:297
#1  0x0000556d659918ee in vga_draw_graphic (full_update=0, s=0x556d68549b30) at /home/stefanb/tmp/qemu-tip/hw/display/vga.c:1695
#2  vga_update_display (opaque=0x556d68549b30) at /home/stefanb/tmp/qemu-tip/hw/display/vga.c:1782
#3  0x0000556d65c0cd92 in vnc_refresh (dcl=0x556d683055a8) at ui/vnc.c:3046
#4  0x0000556d65bff702 in dpy_refresh (s=0x556d686be540) at ui/console.c:1658
#5  gui_update (opaque=0x556d686be540) at ui/console.c:205
#6  0x0000556d65d0deac in timerlist_run_timers (timer_list=0x556d66de0e00) at util/qemu-timer.c:536
#7  0x0000556d65d0e0f7 in qemu_clock_run_timers (type=QEMU_CLOCK_REALTIME) at util/qemu-timer.c:547
#8  qemu_clock_run_all_timers () at util/qemu-timer.c:674
#9  0x0000556d65d0e5d1 in main_loop_wait (nonblocking=<optimized out>) at util/main-loop.c:503
#10 0x0000556d65a5f2ee in main_loop () at vl.c:1865
#11 0x0000556d658ff166 in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at vl.c:4643

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https://bugs.launchpad.net/bugs/1784900

Title:
  QEMU (frontend) crashes upon warm reboot with virtio-gpu device and
  vga=775 on Linux cmdline

Status in QEMU:
  New

Bug description:
  With vga=775 on the Linux command line a first boot of the VM running
  Linux works fine. After a warm reboot it crashes during Linux boot.
  The VM was used remotely via virt-manager and VNC.

  Bisecting the code lead to the following patch that introduced the
  bug:

  commit 1fccd7c5a9a722a9cbf1bc91693f4618034f01ac (HEAD, refs/bisect/bad)
  Author: Gerd Hoffmann <kraxel@redhat.com>
  Date:   Mon Jul 2 18:24:43 2018 +0200

      virtio-gpu: disable scanout when backing resource is destroyed

      Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
      Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
      Message-id: 20180702162443.16796-4-kraxel@redhat.com

  diff --git a/hw/display/virtio-gpu.c b/hw/display/virtio-gpu.c
  index 336dc59007..08cd567218 100644
  --- a/hw/display/virtio-gpu.c
  +++ b/hw/display/virtio-gpu.c
  @@ -430,6 +430,16 @@ static void virtio_gpu_disable_scanout(VirtIOGPU *g, int scanout_id)
   static void virtio_gpu_resource_destroy(VirtIOGPU *g,
                                           struct virtio_gpu_simple_resource *res)
   {
  +    int i;
  +
  +    if (res->scanout_bitmask) {
  +        for (i = 0; i < g->conf.max_outputs; i++) {
  +            if (res->scanout_bitmask & (1 << i)) {
  +                virtio_gpu_disable_scanout(g, i);
  +            }
  +        }
  +    }
  +
       pixman_image_unref(res->image);
       virtio_gpu_cleanup_mapping(res);
       QTAILQ_REMOVE(&g->reslist, res, next);

  
  Reported backtraces can be found here:  https://paste.fedoraproject.org/paste/OUDEfCk1IY7xiy0I0PDlkw

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