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 < 0 && ~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.