In the Linux kernel, the following vulnerability has been resolved:
bpf: Reject tail calls directly from callback frames
A tail call from a non-zero frame is modeled as a return from that frame. The verifier makes R0 unknown and calls prepare_func_exit() for the taken
branch. When the current frame is a synchronous callback, prepare_func_exit()
enforces the callback return-value contract and marks R0 precise.
Since the
tail-call path synthesized R0 rather than deriving it from an instruction,
precision backtracking reaches the callback-calling instruction with R0
still requested and triggers the "callback unexpected regs" verifier bug. A CAP_BPF task can therefore cause a WARN and an -EFAULT BPF_PROG_LOAD. Tail calls reachable from callbacks are already rejected later by
check_max_stack_depth().
Reject a tail call made directly by a callback before constructing the inconsistent return state, using the existing diagnostic. Tail calls from ordinary subprograms keep their current behavior.