From: Daniel Borkmann Date: Wed, 15 Apr 2026 12:14:03 +0000 (+0200) Subject: bpf, arm64: Fix off-by-one in check_imm signed range check X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=1dd8be4ec722ce54e4cace59f3a4ba658111b3ec;p=thirdparty%2Fkernel%2Flinux.git 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. Fixes: e54bcde3d69d ("arm64: eBPF JIT compiler") Signed-off-by: Daniel Borkmann Reviewed-by: Puranjay Mohan Link: https://lore.kernel.org/r/20260415121403.639619-2-daniel@iogearbox.net Signed-off-by: Alexei Starovoitov --- diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c index adf84962d579d..4aad9483f8a50 100644 --- a/arch/arm64/net/bpf_jit_comp.c +++ b/arch/arm64/net/bpf_jit_comp.c @@ -35,8 +35,8 @@ #define ARENA_VM_START (MAX_BPF_JIT_REG + 5) #define check_imm(bits, imm) do { \ - if ((((imm) > 0) && ((imm) >> (bits))) || \ - (((imm) < 0) && (~(imm) >> (bits)))) { \ + if ((((imm) > 0) && ((imm) >> ((bits) - 1))) || \ + (((imm) < 0) && (~(imm) >> ((bits) - 1)))) { \ pr_info("[%2d] imm=%d(0x%x) out of range\n", \ i, imm, imm); \ return -EINVAL; \