CVE-2022-50650 — bpf: Fix reference state management for synchronous callbacks

Affected: Linux

In the Linux kernel, the following vulnerability has been resolved:

bpf: Fix reference state management for synchronous callbacks

Currently, verifier verifies callback functions (sync and async) as if they will be executed once, (i.e. it explores execution state as if the function was being called once). The next insn to explore is set to start of subprog and the exit from nested frame is handled using curframe > 0 and prepare_func_exit. In case of async callback it uses a customized variant of push_stack simulating a kind of branch to set up custom state and execution context for the async callback.

While this approach is simple and works when callback really will be executed only once, it is unsafe for all of our current helpers which are for_each style, i.e. they execute the callback multiple times.

A callback releasing acquired references of the caller may do so multiple times, but currently verifier sees it as one call inside the frame, which then returns to caller. Hence, it thinks it released some reference that the cb e.g. got access through callback_ctx (register filled inside cb from spilled typed register on stack).

Similarly, it may see that an acquire call is unpaired inside the callback, so the caller will copy the reference state of callback and then will have to release the register with new ref_obj_ids. But again, the callback may execute multiple times, but the verifier will only account for acquired references for a single symbolic execution of the ca…

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CVE-2022-50650 — bpf: Fix reference state management for synchronous callbacks