Medium [CVE-2026-53036] bpf, arm64: Fix off-by-one in check_imm signed range check

In the Linux kernel, the following vulnerability has been resolved: bpf, arm64: Fix off-by-one in check_imm signed range check check_imm(bits, imm) is used in the arm64 BPF JIT to verify that a branch displacement (in arm64 instruction units) fits into the signed N-bit immediate field of a B, B.cond or CBZ/CBNZ encoding before it is handed to the encoder.

The macro currently tests for (imm > 0 && imm >> bits) || (imm > bits) which admits values in [-2^N, 2^N) — effectively a signed (N+1)-bit range. A signed N-bit field only holds [-2^(N-1), 2^(N-1)), so the check admits one extra bit of range on each side.

In particular, for check_imm19(), values in [2^18, 2^19) slip past the check but do not fit into the 19-bit signed imm19 field of B.cond. aarch64_insn_encode_immediate() then masks the raw value into the 19-bit field, setting bit 18 (the sign bit) and flipping a forward branch into a backward one.

Same class of issue exists for check_imm26() and the B/BL encoding. Shift by (bits - 1) instead of bits so the actual signed N-bit range is enforced.

Specifically, an off-by-one error exists in the BPF (Berkeley Packet Filter) JIT (Just-In-Time) compiler when handling immediate values for branch instructions on ARM64 architectures. This vulnerability allows the system to process values outside their intended range, which can lead to incorrect program execution flow.