** Tags added: tcg -- You received this bug notification because you are a member of qemu- devel-ml, which is subscribed to QEMU. https://bugs.launchpad.net/bugs/1863025
Title: Use-after-free after flush in TCG accelerator Status in QEMU: Confirmed Bug description: I believe I found a UAF in TCG that can lead to a guest VM escape. The security list informed me "This can not be treated as a security issue." and to post it here. I am looking at the 4.2.0 source code. The issue requires a race and I will try to describe it in terms of three concurrent threads. Thread A: A1. qemu_tcg_cpu_thread_fn runs work loop A2. qemu_wait_io_event => qemu_wait_io_event_common => process_queued_cpu_work A3. start_exclusive critical section entered A4. do_tb_flush is called, TB memory freed/re-allocated A5. end_exclusive exits critical section Thread B: B1. qemu_tcg_cpu_thread_fn runs work loop B2. tcg_cpu_exec => cpu_exec => tb_find => tb_gen_code B3. tcg_tb_alloc obtains a new TB Thread C: C1. qemu_tcg_cpu_thread_fn runs work loop C2. cpu_exec_step_atomic executes C3. TB obtained with tb_lookup__cpu_state or tb_gen_code C4. start_exclusive critical section entered C5. cpu_tb_exec executes the TB code C6. end_exclusive exits critical section Consider the following sequence of events: B2 => B3 => C3 (same TB as B2) => A3 => A4 (TB freed) => A5 => B2 => B3 (re-allocates TB from B2) => C4 => C5 (freed/reused TB now executing) => C6 In short, because thread C uses the TB in the critical section, there is no guarantee that the pointer has not been "freed" (rather the memory is marked as re-usable) and therefore a use-after-free occurs. Since the TCG generated code can be in the same memory as the TB data structure, it is possible for an attacker to overwrite the UAF pointer with code generated from TCG. This can overwrite key pointer values and could lead to code execution on the host outside of the TCG sandbox. To manage notifications about this bug go to: https://bugs.launchpad.net/qemu/+bug/1863025/+subscriptions
