[BUG qemu 4.0] segfault when unplugging virtio-blk-pci device

[Eryu Guan]

Hi,

I'm using qemu 4.0 and hit segfault when tearing down kata sandbox, I
think it's because io completion hits use-after-free when device is
already gone. Is this a known bug that has been fixed? (I went through
the git log but didn't find anything obvious).

gdb backtrace is:

Core was generated by `/usr/local/libexec/qemu-kvm -name 
sandbox-5b8df8c6c6901c3c0a9b02879be10fe8d69d6'.
Program terminated with signal 11, Segmentation fault.
#0 object_get_class (obj=obj@entry=0x0) at 
/usr/src/debug/qemu-4.0/qom/object.c:903
903        return obj->class;
(gdb) bt
#0  object_get_class (obj=obj@entry=0x0) at 
/usr/src/debug/qemu-4.0/qom/object.c:903
#1  0x0000558a2c009e9b in virtio_notify_vector (vdev=0x558a2e7751d0,
    vector=<optimized out>) at /usr/src/debug/qemu-4.0/hw/virtio/virtio.c:1118
#2  0x0000558a2bfdcb1e in virtio_blk_discard_write_zeroes_complete (
    opaque=0x558a2f2fd420, ret=0)
    at /usr/src/debug/qemu-4.0/hw/block/virtio-blk.c:186
#3  0x0000558a2c261c7e in blk_aio_complete (acb=0x558a2eed7420)
    at /usr/src/debug/qemu-4.0/block/block-backend.c:1305
#4  0x0000558a2c3031db in coroutine_trampoline (i0=<optimized out>,
    i1=<optimized out>) at /usr/src/debug/qemu-4.0/util/coroutine-ucontext.c:116
#5  0x00007f45b2f8b080 in ?? () from /lib64/libc.so.6
#6  0x00007fff9ed75780 in ?? ()
#7  0x0000000000000000 in ?? ()

It seems like qemu was completing a discard/write_zero request, but
parent BusState was already freed & set to NULL.

Do we need to drain all pending request before unrealizing virtio-blk
device? Like the following patch proposed?

https://lists.gnu.org/archive/html/qemu-devel/2017-06/msg02945.html

If more info is needed, please let me know.

Thanks,
Eryu