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

Official description

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.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/a5dfeb3b61065039488342d43ae06d4729d955d4
• https://git.kernel.org/stable/c/7fd3b41260c6120e7b60164afea5d961af6224f9
• https://git.kernel.org/stable/c/6927f0d6794aa73318bbfa929f1ff6065b0620df
• https://git.kernel.org/stable/c/1a113b5497297871699cd498b1b83542e0db7f15
• https://git.kernel.org/stable/c/fb74defa1cca1a73177c0c761e641332e4f979a3
• https://git.kernel.org/stable/c/1dd8be4ec722ce54e4cace59f3a4ba658111b3ec