| Message ID | 20240119233922.32588-2-farosas@suse.de |
|---|---|
| State | New |
| Headers | show |
| Series | migration: Fix migration state reference counting | expand |
Fabiano Rosas <farosas@suse.de> writes: > 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> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1969
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()?
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()? 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)? (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.
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. > (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.
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.
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:
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(+)