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migration: Fix use-after-free of migration state object
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(). Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1969 Signed-off-by: Fabiano Rosas <farosas@suse.de> Link: https://lore.kernel.org/r/20240119233922.32588-2-farosas@suse.de Signed-off-by: Peter Xu <peterx@redhat.com> (cherry picked from commit 27eb849) Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
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