null pointer access on migration resume of systemrescuecd boot menu with qxl-vga

[Matthew Stapleton]

With qemu-2.8.0 up to 2.9.0-rc2 and git master (6954cdc), when resuming from a migration state file created from a VM suspended while showing the System Rescue CD 4.9.2 boot menu and using the QXL VGA device, I get a null point access in pixman_image_get_data called from qemu_spice_create_update (spice-display.c:215). When I added assert(ssd->mirror != NULL) above that line, assert failed. I don't get the crash when using standard VGA or cirrus-vga. I am using gcc-4.9.3 on Gentoo x86_64 with Intel i7-4700HQ CPU and kernel: 4.9.15-gentoo.

Here is the valgrind trace from the git version:
==2634== Thread 1:
==2634== Invalid read of size 4
==3516== at 0x65F3050: pixman_image_get_data (in /usr/lib64/libpixman-1.so.0.34.0)
==3516== by 0x6F0CEB: qemu_spice_create_update (spice-display.c:215)
==3516== by 0x6F1CC7: qemu_spice_display_refresh (spice-display.c:502)
==3516== by 0x58CF77: display_refresh (qxl.c:1948)
==3516== by 0x6E8084: do_safe_dpy_refresh (console.c:1591)
==3516== by 0x6E80D5: dpy_refresh (console.c:1604)
==3516== by 0x6E4508: gui_update (console.c:201)
==3516== by 0x81898E: timerlist_run_timers (qemu-timer.c:536)
==3516== by 0x8189D6: qemu_clock_run_timers (qemu-timer.c:547)
==3516== by 0x818D98: qemu_clock_run_all_timers (qemu-timer.c:662)
==3516== by 0x81952A: main_loop_wait (main-loop.c:514)
==3516== by 0x4ADD29: main_loop (vl.c:1898)

Minimal steps to reproduce:

Compile (debug compile flags are just so valgrind works, the crash occurs with non-debug compile flags as well):
CFLAGS="-g -O0" CXXFLAGS="-g -O0" ./configure --target-list=i386-softmmu,x86_64-softmmu
./configure
make

Start VM and leave it on the System Rescue CD graphical boot menu:
x86_64-softmmu/qemu-system-x86_64 -nodefaults -machine pc -drive file=systemrescuecd-x86-4.9.2.iso,if=none,id=cdrom-cd,readonly=on -device ide-cd,bus=ide.0,drive=cdrom-cd,bootindex=1 -device qxl-vga -monitor unix:monitor.sock,server,nowait -display gtk

Suspend VM and save state:
socat - unix:monitor.sock
  stop
  migrate "exec:cat > vm.state"
  quit

Attempt to resume VM (but this crashes):
x86_64-softmmu/qemu-system-x86_64 -nodefaults -machine pc -drive file=systemrescuecd-x86-4.9.2.iso,if=none,id=cdrom-cd,readonly=on -device ide-cd,bus=ide.0,drive=cdrom-cd,bootindex=1 -device qxl-vga -monitor unix:monitor.sock,server,nowait -display gtk -incoming exec:"cat vm.state"

[Dr. David Alan Gilbert (dgilbert-h)]

Yep, I can repeat this here on qemu head; crash at:

pixman_image_get_data (image=0x0) at pixman-image.c:845
845 if (image->type == BITS)

(gdb) p image
$1 = (pixman_image_t *) 0x0

[Dr. David Alan Gilbert (dgilbert-h)]

I think this is actually anything that's in text mode grub; I've had a RHEL5 and 6 VM do it as well.

Thanks for reporting it.

[elmarco (marcandre-lureau)]

Interesting, the culprit is:

commit cd958edb1fae85d0c7d1e1acbff82d22724e8d64
Author: Marc-André Lureau <email address hidden>
Date: Fri Aug 26 13:47:11 2016 +0400

    console: skip same-size resize

    virtio-gpu does a set-scanout at each frame (it might be a driver
    regression). qemu_console_resize() recreate a surface even if the size
    didn't change, and this shows up in profiling reports because the
    surface is cleared. With this patch, I get a +15-20% glmark2
    improvement.

    Signed-off-by: Marc-André Lureau <email address hidden>
    Message-id: <email address hidden>
    Signed-off-by: Gerd Hoffmann <email address hidden>

diff --git a/ui/console.c b/ui/console.c
index 3940762851..394786b3c7 100644
--- a/ui/console.c
+++ b/ui/console.c
@@ -2101,6 +2101,13 @@ void qemu_console_resize(QemuConsole *s, int width, int height)
     DisplaySurface *surface;

     assert(s->console_type == GRAPHIC_CONSOLE);
+
+ if (s->surface &&
+ pixman_image_get_width(s->surface->image) == width &&
+ pixman_image_get_height(s->surface->image) == height) {
+ return;
+ }

[Thomas Huth (th-huth)]

The fix has apparently been included here:
https://git.qemu.org/?p=qemu.git;a=commitdiff;h=a703d3aef5991b72a5a45880e7491232b8032f09
... and has been released with QEMU v2.9 already.