Re: [Qemu-devel] [PATCH] migration: fix migration shutdown

[Yury Kotov]

03.04.2019, 22:06, "Dr. David Alan Gilbert" <address@hidden>:
> * Yury Kotov (address@hidden) wrote:
>>  It fixes heap-use-after-free which was found by clang's ASAN.
>>
>>  Control flow of this use-after-free:
>>  main_thread:
>>      * Got SIGTERM and completes main loop
>>      * Calls migration_shutdown
>>        - migrate_fd_cancel (so, migration_thread begins to complete)
>>        - object_unref(OBJECT(current_migration));
>>
>>  migration_thread:
>>      * migration_iteration_finish -> schedule cleanup bh
>>      * object_unref(OBJECT(s)); (Now, current_migration is freed)
>>      * exits
>>
>>  main_thread:
>>      * Calls vm_shutdown -> drain bdrvs -> main loop
>>        -> cleanup_bh -> use after free
>>
>>  If you want to reproduce, these couple of sleeps will help:
>>  vl.c:4613:
>>       migration_shutdown();
>>  + sleep(2);
>>  migration.c:3269:
>>  + sleep(1);
>>       trace_migration_thread_after_loop();
>>       migration_iteration_finish(s);
>
> Fun; I hadn't realised you could get more main loop after exiting
> the main loop.
>
> Is this fallout from my 892ae715b6b or is this just another case it
> didn't catch?
> I guess it moved the shutdown early, so it probably is fallout, and
> so probably does need to go to 4.0 ?
>
I think this is closer to another case than fallout. Because without
892ae715b6b, reproducing of UAF at exit was much easier.
Another way to fix it - remove unref from migration_shutdown
(but keep migrate_fd_cancel) and restore migration_object_finalize.

>>  Original output:
>>  qemu-system-x86_64: terminating on signal 15 from pid 31980 (<unknown 
>> process>)
>>  =================================================================
>>  ==31958==ERROR: AddressSanitizer: heap-use-after-free on address 
>> 0x61900001d210
>>    at pc 0x555558a535ca bp 0x7fffffffb190 sp 0x7fffffffb188
>>  READ of size 8 at 0x61900001d210 thread T0 (qemu-vm-0)
>>      #0 0x555558a535c9 in migrate_fd_cleanup migration/migration.c:1502:23
>>      #1 0x5555594fde0a in aio_bh_call util/async.c:90:5
>>      #2 0x5555594fe522 in aio_bh_poll util/async.c:118:13
>>      #3 0x555559524783 in aio_poll util/aio-posix.c:725:17
>>      #4 0x555559504fb3 in aio_wait_bh_oneshot util/aio-wait.c:71:5
>>      #5 0x5555573bddf6 in virtio_blk_data_plane_stop
>>        hw/block/dataplane/virtio-blk.c:282:5
>>      #6 0x5555589d5c09 in virtio_bus_stop_ioeventfd 
>> hw/virtio/virtio-bus.c:246:9
>>      #7 0x5555589e9917 in virtio_pci_stop_ioeventfd 
>> hw/virtio/virtio-pci.c:287:5
>>      #8 0x5555589e22bf in virtio_pci_vmstate_change 
>> hw/virtio/virtio-pci.c:1072:9
>>      #9 0x555557628931 in virtio_vmstate_change hw/virtio/virtio.c:2257:9
>>      #10 0x555557c36713 in vm_state_notify vl.c:1605:9
>>      #11 0x55555716ef53 in do_vm_stop cpus.c:1074:9
>>      #12 0x55555716eeff in vm_shutdown cpus.c:1092:12
>>      #13 0x555557c4283e in main vl.c:4617:5
>>      #14 0x7fffdfdb482f in __libc_start_main
>>        (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
>>      #15 0x555556ecb118 in _start 
>> (x86_64-softmmu/qemu-system-x86_64+0x1977118)
>>
>>  0x61900001d210 is located 144 bytes inside of 952-byte region
>>    [0x61900001d180,0x61900001d538)
>>  freed by thread T6 (live_migration) here:
>>      #0 0x555556f76782 in __interceptor_free
>>        
>> /tmp/final/llvm.src/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:124:3
>>      #1 0x555558d5fa94 in object_finalize qom/object.c:618:9
>>      #2 0x555558d57651 in object_unref qom/object.c:1068:9
>>      #3 0x555558a55588 in migration_thread migration/migration.c:3272:5
>>      #4 0x5555595393f2 in qemu_thread_start util/qemu-thread-posix.c:502:9
>>      #5 0x7fffe057f6b9 in start_thread 
>> (/lib/x86_64-linux-gnu/libpthread.so.0+0x76b9)
>>
>>  previously allocated by thread T0 (qemu-vm-0) here:
>>      #0 0x555556f76b03 in __interceptor_malloc
>>        
>> /tmp/final/llvm.src/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:146:3
>>      #1 0x7ffff6ee37b8 in g_malloc 
>> (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x4f7b8)
>>      #2 0x555558d58031 in object_new qom/object.c:640:12
>>      #3 0x555558a31f21 in migration_object_init migration/migration.c:139:25
>>      #4 0x555557c41398 in main vl.c:4320:5
>>      #5 0x7fffdfdb482f in __libc_start_main 
>> (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
>>
>>  Thread T6 (live_migration) created by T0 (qemu-vm-0) here:
>>      #0 0x555556f5f0dd in pthread_create
>>        
>> /tmp/final/llvm.src/projects/compiler-rt/lib/asan/asan_interceptors.cc:210:3
>>      #1 0x555559538cf9 in qemu_thread_create util/qemu-thread-posix.c:539:11
>>      #2 0x555558a53304 in migrate_fd_connect migration/migration.c:3332:5
>>      #3 0x555558a72bd8 in migration_channel_connect migration/channel.c:92:5
>>      #4 0x555558a6ef87 in exec_start_outgoing_migration migration/exec.c:42:5
>>      #5 0x555558a4f3c2 in qmp_migrate migration/migration.c:1922:9
>>      #6 0x555558bb4f6a in qmp_marshal_migrate 
>> qapi/qapi-commands-migration.c:607:5
>>      #7 0x555559363738 in do_qmp_dispatch qapi/qmp-dispatch.c:131:5
>>      #8 0x555559362a15 in qmp_dispatch qapi/qmp-dispatch.c:174:11
>>      #9 0x5555571bac15 in monitor_qmp_dispatch monitor.c:4124:11
>>      #10 0x55555719a22d in monitor_qmp_bh_dispatcher monitor.c:4207:9
>>      #11 0x5555594fde0a in aio_bh_call util/async.c:90:5
>>      #12 0x5555594fe522 in aio_bh_poll util/async.c:118:13
>>      #13 0x5555595201e0 in aio_dispatch util/aio-posix.c:460:5
>>      #14 0x555559503553 in aio_ctx_dispatch util/async.c:261:5
>>      #15 0x7ffff6ede196 in g_main_context_dispatch
>>        (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x4a196)
>>
>>  SUMMARY: AddressSanitizer: heap-use-after-free migration/migration.c:1502:23
>>    in migrate_fd_cleanup
>>  Shadow bytes around the buggy address:
>>    0x0c327fffb9f0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
>>    0x0c327fffba00: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
>>    0x0c327fffba10: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
>>    0x0c327fffba20: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
>>    0x0c327fffba30: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
>>  =>0x0c327fffba40: fd fd[fd]fd fd fd fd fd fd fd fd fd fd fd fd fd
>>    0x0c327fffba50: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
>>    0x0c327fffba60: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
>>    0x0c327fffba70: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
>>    0x0c327fffba80: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
>>    0x0c327fffba90: 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
>>    Freed heap region: fd
>>    Stack left redzone: f1
>>    Stack mid redzone: f2
>>    Stack right redzone: f3
>>    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
>>    Left alloca redzone: ca
>>    Right alloca redzone: cb
>>    Shadow gap: cc
>>  ==31958==ABORTING
>>
>>  Signed-off-by: Yury Kotov <address@hidden>
>>  ---
>>   migration/migration.c | 30 ++++++++++++++++++++++++------
>>   1 file changed, 24 insertions(+), 6 deletions(-)
>>
>>  diff --git a/migration/migration.c b/migration/migration.c
>>  index 609e0df5d0..54d82bb88b 100644
>>  --- a/migration/migration.c
>>  +++ b/migration/migration.c
>>  @@ -128,6 +128,8 @@ static int migration_maybe_pause(MigrationState *s,
>>                                    int *current_active_state,
>>                                    int new_state);
>>   static void migrate_fd_cancel(MigrationState *s);
>>  +static void migrate_join_thread(MigrationState *s);
>>  +static void migrate_fd_cleanup(void *opaque);
>>
>>   void migration_object_init(void)
>>   {
>>  @@ -176,6 +178,17 @@ void migration_shutdown(void)
>>        * stop the migration using this structure
>>        */
>>       migrate_fd_cancel(current_migration);
>>  + /*
>>  + * migration_thread schedules cleanup_bh, but migration_shutdown is called
>>  + * from the end of main, so cleanu_up may not be called because main-loop 
>> is
>>  + * complete. So, wait for the complition of migration_thread, cancel bh and
>>  + * call cleanup ourselves.
>>  + */
>
> (Some typos)
>
Ok :)

>>  + migrate_join_thread(current_migration);
>
> I'll admit to being a bit nervous that something in here could be
> hanging and thus we could hang here rather than die as SIGTERM is
> expecting us to.
>
I think it's only way to be sure we havn't other races with migration_thread.
And this joining is cheaper than vm_shutdown IMHO.

>>  + if (current_migration->cleanup_bh) {
>>  + qemu_bh_cancel(current_migration->cleanup_bh);
>
> Is that needed? Doesn't migrate_fd_cleanup start with a qemu_bh_delete?
>
Agree, it's odd.

>>  + migrate_fd_cleanup(current_migration);
>>  + }
>>       object_unref(OBJECT(current_migration));
>>   }
>>
>>  @@ -1495,6 +1508,16 @@ static void block_cleanup_parameters(MigrationState 
>> *s)
>>       }
>>   }
>>
>>  +static void migrate_join_thread(MigrationState *s)
>>  +{
>>  + qemu_mutex_unlock_iothread();
>>  + if (s->migration_thread_running) {
>>  + qemu_thread_join(&s->thread);
>>  + s->migration_thread_running = false;
>>  + }
>>  + qemu_mutex_lock_iothread();
>>  +}
>>  +
>>   static void migrate_fd_cleanup(void *opaque)
>>   {
>>       MigrationState *s = opaque;
>>  @@ -1508,12 +1531,7 @@ static void migrate_fd_cleanup(void *opaque)
>>           QEMUFile *tmp;
>>
>>           trace_migrate_fd_cleanup();
>>  - qemu_mutex_unlock_iothread();
>>  - if (s->migration_thread_running) {
>>  - qemu_thread_join(&s->thread);
>>  - s->migration_thread_running = false;
>>  - }
>>  - qemu_mutex_lock_iothread();
>>  + migrate_join_thread(s);
>>
>>           multifd_save_cleanup();
>>           qemu_mutex_lock(&s->qemu_file_lock);
>
> Dave
>
>>  --
>>  2.21.0
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK