[Qemu-devel] [PATCH] virtio-blk: drain block before cleanup

[Gioh Kim <gi-oh.kim@profitbricks.com>]

Hi,

I'd like to report one use-after-free problem which is found by AddressSanitizer.
My company provides virtualization server with Qemu-2.7.
If customer commands Disk hot-unplug on Web-based application,
the application sends "device_del" and "drive_del" commands to qemu process via QMP.
It usually works fine.
But I found a corner case like following.
1. Customer commands Disk hot-unplug via Web-based application.
2. The application sends "device_del" that sometimes takes unpredictable time.
3. There are some inflight IOs
4. Customer does reboot or shutdown the guest OS
4. Qemu process generates segfault.

If a disk is unplugged with "device_del" and "drive_del" commands,
qemu does not generate problem. But if only "device_del" command are finished and
guest os reboots, qemu generates segfault.
I can reproduce that case easily with following commands
1. generate heavy IO on disk in Guest OS
2. run "device_del" on the qemu monitor
3. run "system_reboot" on the qemu monitor

According to AdressSanitizer, VirtQueue is freed by virtio_cleanup but dereferenced
by asynchronous request. I think asynchronous requests should be finished
before virtio_cleanup, so I added blk_drain() before virtio_cleanup.

Following is configure options I used to build qemu with AddressSanitizer.
./configure --target-list=x86_64-softmmu --extra-cflags="-fsanitize=address -fno-omit-frame-pointer" --enable-debug

Following is the report of AddressSanitizer.

------------------------------------- 8< ----------------------------------------
==8801==WARNING: ASan doesn't fully support makecontext/swapcontext functions and may produce false positives in some cases!
==8801==WARNING: ASan is ignoring requested __asan_handle_no_return: stack top: 0x7ffc984da000; bottom 0x7fa524363000; size: 0x005774177000 (375609847808)
False positive error reports may follow
For details see http://code.google.com/p/address-sanitizer/issues/detail?id=189
=================================================================
==8801==ERROR: AddressSanitizer: heap-use-after-free on address 0x7fa5240ab82c at pc 0x55ccd7bffb76 bp 0x7fa524364590 sp 0x7fa524364580
READ of size 2 at 0x7fa5240ab82c thread T0
    #0 0x55ccd7bffb75 in virtqueue_fill /home/gohkim/work/tools/qemu/hw/virtio/virtio.c:284
    #1 0x55ccd7bffe46 in virtqueue_push /home/gohkim/work/tools/qemu/hw/virtio/virtio.c:308
    #2 0x55ccd7b66c74 in virtio_blk_req_complete /home/gohkim/work/tools/qemu/hw/block/virtio-blk.c:58
    #3 0x55ccd7b67154 in virtio_blk_rw_complete /home/gohkim/work/tools/qemu/hw/block/virtio-blk.c:121
    #4 0x55ccd83852b9 in blk_aio_complete block/block-backend.c:923
    #5 0x55ccd8385a73 in blk_aio_write_entry block/block-backend.c:984
    #6 0x55ccd84c7d68 in coroutine_trampoline util/coroutine-ucontext.c:78
    #7 0x7fa5702d45cf  (/lib/x86_64-linux-gnu/libc.so.6+0x495cf)

0x7fa5240ab82c is located 44 bytes inside of 131072-byte region [0x7fa5240ab800,0x7fa5240cb800)
freed by thread T0 here:
    #0 0x7fa573f876aa in __interceptor_free (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x986aa)
    #1 0x55ccd7c07fa4 in virtio_cleanup /home/gohkim/work/tools/qemu/hw/virtio/virtio.c:1678
    #2 0x55ccd7b6c585 in virtio_blk_device_unrealize /home/gohkim/work/tools/qemu/hw/block/virtio-blk.c:948
    #3 0x55ccd7c09734 in virtio_device_unrealize /home/gohkim/work/tools/qemu/hw/virtio/virtio.c:1950
    #4 0x55ccd7eb9bb7 in device_set_realized hw/core/qdev.c:964
    #5 0x55ccd82c1bef in property_set_bool qom/object.c:1853
    #6 0x55ccd82be535 in object_property_set qom/object.c:1087
    #7 0x55ccd82c454f in object_property_set_qobject qom/qom-qobject.c:27
    #8 0x55ccd82be810 in object_property_set_bool qom/object.c:1156
    #9 0x55ccd7ec1395 in bus_set_realized hw/core/bus.c:181
    #10 0x55ccd82c1bef in property_set_bool qom/object.c:1853
    #11 0x55ccd82be535 in object_property_set qom/object.c:1087
    #12 0x55ccd82c454f in object_property_set_qobject qom/qom-qobject.c:27
    #13 0x55ccd82be810 in object_property_set_bool qom/object.c:1156
    #14 0x55ccd7eb9a8a in device_set_realized hw/core/qdev.c:956
    #15 0x55ccd82c1bef in property_set_bool qom/object.c:1853
    #16 0x55ccd82be535 in object_property_set qom/object.c:1087
    #17 0x55ccd82c454f in object_property_set_qobject qom/qom-qobject.c:27
    #18 0x55ccd82be810 in object_property_set_bool qom/object.c:1156
    #19 0x55ccd7eba820 in device_unparent hw/core/qdev.c:1099
    #20 0x55ccd82bf50b in object_finalize_child_property qom/object.c:1362
    #21 0x55ccd82bb3c6 in object_property_del_child qom/object.c:422
    #22 0x55ccd82bb601 in object_unparent qom/object.c:441
    #23 0x55ccd7e221f1 in acpi_pcihp_eject_slot hw/acpi/pcihp.c:139
    #24 0x55ccd7e22301 in acpi_pcihp_update_hotplug_bus hw/acpi/pcihp.c:152
    #25 0x55ccd7e22600 in acpi_pcihp_update hw/acpi/pcihp.c:176
    #26 0x55ccd7e22628 in acpi_pcihp_reset hw/acpi/pcihp.c:182
    #27 0x55ccd7e1fc10 in piix4_reset hw/acpi/piix4.c:363
    #28 0x55ccd7da4ebd in qemu_devices_reset /home/gohkim/work/tools/qemu/vl.c:1713
    #29 0x55ccd7c28354 in pc_machine_reset /home/gohkim/work/tools/qemu/hw/i386/pc.c:2178

previously allocated by thread T0 here:
    #0 0x7fa573f87b49 in __interceptor_calloc (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x98b49)
    #1 0x7fa5718575d0 in g_malloc0 (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x4f5d0)
    #2 0x55ccd7b6bfa5 in virtio_blk_device_realize /home/gohkim/work/tools/qemu/hw/block/virtio-blk.c:910
    #3 0x55ccd7c09500 in virtio_device_realize /home/gohkim/work/tools/qemu/hw/virtio/virtio.c:1927
    #4 0x55ccd7eb9690 in device_set_realized hw/core/qdev.c:918
    #5 0x55ccd82c1bef in property_set_bool qom/object.c:1853
    #6 0x55ccd82be535 in object_property_set qom/object.c:1087
    #7 0x55ccd82c454f in object_property_set_qobject qom/qom-qobject.c:27
    #8 0x55ccd82be810 in object_property_set_bool qom/object.c:1156
    #9 0x55ccd81b1b5c in virtio_blk_pci_realize hw/virtio/virtio-pci.c:1897
    #10 0x55ccd81b14df in virtio_pci_realize hw/virtio/virtio-pci.c:1799
    #11 0x55ccd805e2c5 in pci_qdev_realize hw/pci/pci.c:1966
    #12 0x55ccd81b17db in virtio_pci_dc_realize hw/virtio/virtio-pci.c:1852
    #13 0x55ccd7eb9690 in device_set_realized hw/core/qdev.c:918
    #14 0x55ccd82c1bef in property_set_bool qom/object.c:1853
    #15 0x55ccd82be535 in object_property_set qom/object.c:1087
    #16 0x55ccd82c454f in object_property_set_qobject qom/qom-qobject.c:27
    #17 0x55ccd82be810 in object_property_set_bool qom/object.c:1156
    #18 0x55ccd7d8141e in qdev_device_add /home/gohkim/work/tools/qemu/qdev-monitor.c:618
    #19 0x55ccd7dab462 in device_init_func /home/gohkim/work/tools/qemu/vl.c:2316
    #20 0x55ccd84bd18c in qemu_opts_foreach util/qemu-option.c:1116
    #21 0x55ccd7db2bdb in main /home/gohkim/work/tools/qemu/vl.c:4507
    #22 0x7fa5702ababf in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x20abf)

SUMMARY: AddressSanitizer: heap-use-after-free /home/gohkim/work/tools/qemu/hw/virtio/virtio.c:284 virtqueue_fill
Shadow bytes around the buggy address:
  0x0ff52480d6b0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0ff52480d6c0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0ff52480d6d0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0ff52480d6e0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0ff52480d6f0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
=>0x0ff52480d700: fd fd fd fd fd[fd]fd fd fd fd fd fd fd fd fd fd
  0x0ff52480d710: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0ff52480d720: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0ff52480d730: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0ff52480d740: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0ff52480d750: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Heap right redzone:      fb
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack partial redzone:   f4
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
==8801==ABORTING


Signed-off-by: Gioh Kim <gi-oh.kim@profitbricks.com>
---
 hw/block/virtio-blk.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index 331d766..cc39934 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -939,6 +939,8 @@ static void virtio_blk_device_unrealize(DeviceState *dev, Error **errp)
     virtio_blk_data_plane_destroy(s->dataplane);
     s->dataplane = NULL;
     qemu_del_vm_change_state_handler(s->change);
+    if (blk_bs(s->blk) && bdrv_requests_pending((blk_bs(s->blk))))
+        blk_drain(s->blk);
     blockdev_mark_auto_del(s->blk);
     virtio_cleanup(vdev);
 }
-- 
2.5.0