Re: [PATCH 1/5] migration: Fix use-after-free of migration state object

[Peter Xu <peterx@redhat.com>]

On Mon, Jan 22, 2024 at 01:55:45PM -0300, Fabiano Rosas wrote:
> Peter Xu <peterx@redhat.com> writes:
> 
> > On Mon, Jan 22, 2024 at 05:49:01PM +0800, Peter Xu wrote:
> >> On Fri, Jan 19, 2024 at 08:39:18PM -0300, Fabiano Rosas wrote:
> >> > We're currently allowing the process_incoming_migration_bh bottom-half
> >> > to run without holding a reference to the 'current_migration' object,
> >> > which leads to a segmentation fault if the BH is still live after
> >> > migration_shutdown() has dropped the last reference to
> >> > current_migration.
> >> > 
> >> > In my system the bug manifests as migrate_multifd() returning true
> >> > when it shouldn't and multifd_load_shutdown() calling
> >> > multifd_recv_terminate_threads() which crashes due to an uninitialized
> >> > multifd_recv_state.
> >> > 
> >> > Fix the issue by holding a reference to the object when scheduling the
> >> > BH and dropping it before returning from the BH. The same is already
> >> > done for the cleanup_bh at migrate_fd_cleanup_schedule().
> >> > 
> >> > Signed-off-by: Fabiano Rosas <farosas@suse.de>
> >> > ---
> >> >  migration/migration.c | 2 ++
> >> >  1 file changed, 2 insertions(+)
> >> > 
> >> > diff --git a/migration/migration.c b/migration/migration.c
> >> > index 219447dea1..cf17b68e57 100644
> >> > --- a/migration/migration.c
> >> > +++ b/migration/migration.c
> >> > @@ -648,6 +648,7 @@ static void process_incoming_migration_bh(void *opaque)
> >> >                        MIGRATION_STATUS_COMPLETED);
> >> >      qemu_bh_delete(mis->bh);
> >> >      migration_incoming_state_destroy();
> >> > +    object_unref(OBJECT(migrate_get_current()));
> >> >  }
> >> >  
> >> >  static void coroutine_fn
> >> > @@ -713,6 +714,7 @@ process_incoming_migration_co(void *opaque)
> >> >      }
> >> >  
> >> >      mis->bh = qemu_bh_new(process_incoming_migration_bh, mis);
> >> > +    object_ref(OBJECT(migrate_get_current()));
> >> >      qemu_bh_schedule(mis->bh);
> >> >      return;
> >> >  fail:
> >> > -- 
> >> > 2.35.3
> >> > 
> >> 
> > I know I missed something, but I'd better ask: use-after-free needs to
> > happen only after migration_shutdown() / qemu_cleanup(), am I right?
> > 
> > If so, shouldn't qemu_main_loop() already returned?  Then how could any BH
> > keep running (including migration's) without qemu_main_loop()?
> 
> The aio_bh_poll() -> aio_bh_call() sequence doesn't depend on
> qemu_main_loop(). In the stack you found below it happens after
> aio_wait_bh_oneshot() -> AIO_WAIT_WHILE -> aio_poll().
> 
> The stack I see is:
> 
> #0  0x00005625c97bffc0 in multifd_recv_terminate_threads (err=0x0) at ../migration/multifd.c:992
> #1  0x00005625c97c0086 in multifd_load_shutdown () at ../migration/multifd.c:1012
> #2  0x00005625c97b6163 in process_incoming_migration_bh (opaque=0x5625cbce59f0) at ../migration/migration.c:624
> #3  0x00005625c9c740c2 in aio_bh_call (bh=0x5625cc9e1320) at ../util/async.c:169
> #4  0x00005625c9c741de in aio_bh_poll (ctx=0x5625cba2a670) at ../util/async.c:216
> 
> here^
> 
> #5  0x00005625c9af0599 in bdrv_graph_wrunlock () at ../block/graph-lock.c:170
> #6  0x00005625c9aba8bd in bdrv_close (bs=0x5625cbcb3d80) at ../block.c:5099
> #7  0x00005625c9abb71a in bdrv_delete (bs=0x5625cbcb3d80) at ../block.c:5501
> #8  0x00005625c9abe840 in bdrv_unref (bs=0x5625cbcb3d80) at ../block.c:7075
> #9  0x00005625c9abe865 in bdrv_schedule_unref_bh (opaque=0x5625cbcb3d80) at ../block.c:7083
> #10 0x00005625c9c740c2 in aio_bh_call (bh=0x5625cbde09d0) at ../util/async.c:169
> #11 0x00005625c9c741de in aio_bh_poll (ctx=0x5625cba2a670) at ../util/async.c:216
> #12 0x00005625c9af0599 in bdrv_graph_wrunlock () at ../block/graph-lock.c:170
> #13 0x00005625c9ae05db in blk_remove_bs (blk=0x5625cbcc1070) at ../block/block-backend.c:907
> #14 0x00005625c9adfb1b in blk_remove_all_bs () at ../block/block-backend.c:571
> #15 0x00005625c9abab0d in bdrv_close_all () at ../block.c:5146
> #16 0x00005625c97892e4 in qemu_cleanup (status=0) at ../system/runstate.c:880
> #17 0x00005625c9a58081 in qemu_default_main () at ../system/main.c:38
> #18 0x00005625c9a580af in main (argc=35, argv=0x7ffe30ab0578) at ../system/main.c:48
> 
> > Hmm, I saw a pretty old stack mentioned in commit fd392cfa8e6:
> >
> >     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)
> >
> > Would that be the same case that you mentioned here?  As vm_shutdown() is
> > indeed after migration_shutdown().
> >
> > Even if so, two follow up comments..
> >
> > (1) How did it help if process_incoming_migration_bh() took ref on
> >     MigrationState*?  I didn't even see it touching the object (instead, it
> >     uses the incoming object)?
> 
> We touch MigrationState every time we check for a capability. See the
> stack I posted above: process_incoming_migration_bh() ->
> multifd_load_shutdown().
> 
> void multifd_load_shutdown(void)
> {
>     if (migrate_multifd()) {        <-- HERE
>         multifd_recv_terminate_threads(NULL);
>     }
> }
> 
> The bug reproduces *without* multifd, because that check passes and we
> go into multifd code that has not been initialized.
> 
> side note: we should probably introduce a MigrationOutgoingState to pair
> with MigrationIncomingState and have both inside a global MigrationState
> that contains the common elements. If you agree I can add this to our
> todo list.

Yep, feel free to add an entry.

Though instead of MigrationOutgoingState, maybe it's easier to keep
MigrationState to be the "outgoing state", then move both caps/params (and
anything shared between src/dst) out of MigrationState?  That can be a
separate struct, e.g., MigrationGlobals.

> 
> > (2) This is what I'm just wondering.. on whether we should clear
> >     current_migration to NULL in migration_shutdown() after we unref it.
> >     Maybe it'll make such issues abort in an even clearer way.
> 
> It hits the assert at migrate_get_current():
> 
> #4  0x00005643006e22ae in migrate_get_current () at ../migration/migration.c:246
> #5  0x00005643006f0415 in migrate_late_block_activate () at ../migration/options.c:275
> #6  0x00005643006e30e0 in process_incoming_migration_bh (opaque=0x564303b279f0) at ../migration/migration.c:603
> #7  0x0000564300ba10cd in aio_bh_call (bh=0x564304823320) at ../util/async.c:169
> #8  0x0000564300ba11e9 in aio_bh_poll (ctx=0x56430386c670) at ../util/async.c:216
> ...
> #20 0x00005643006b62e4 in qemu_cleanup (status=0) at ../system/runstate.c:880
> #21 0x000056430098508c in qemu_default_main () at ../system/main.c:38
> #22 0x00005643009850ba in main (argc=35, argv=0x7ffc8bf703c8) at
> #../system/main.c:4
> 
> Note that this breaks at migrate_late_block_activate(), which is even
> earlier than the bug scenario at multifd_load_shutdown().
> 
> However we cannot set NULL currently because the BHs are still running
> after migration_shutdown(). I don't know of a safe way to cancel/delete
> a BH after it has (potentially) been scheduled already.

Indeed..  I am not sure whether migration can avoid using BHs some day.
E.g., I am not sure whether those tasks can be done synchronously under bql
already.

-- 
Peter Xu