bpf, arm64: Reject out-of-range B.cond targets

aarch64_insn_gen_cond_branch_imm() calls label_imm_common() to
compute a 19-bit signed byte offset for a conditional branch,
but unlike its siblings aarch64_insn_gen_branch_imm() and
aarch64_insn_gen_comp_branch_imm(), it does not check whether
label_imm_common() returned its out-of-range sentinel (range)
before feeding the value to aarch64_insn_encode_immediate().

aarch64_insn_encode_immediate() unconditionally masks the value
with the 19-bit field mask, so an offset that was rejected by
label_imm_common() gets silently truncated. With the sentinel
value SZ_1M, the resulting field ends up with bit 18 (the sign
bit of the 19-bit signed displacement) set, and the CPU decodes
it as a ~1 MiB *backward* branch, producing an incorrectly
targeted B.cond instruction. For code-gen locations like the
emit_bpf_tail_call() this function is the only barrier between
an overflowing displacement and a silently miscompiled branch.

Fix it by returning AARCH64_BREAK_FAULT when the offset is out
of range, so callers see a loud failure instead of a silently
misencoded branch. validate_code() scans the generated image
for any AARCH64_BREAK_FAULT and then lets the JIT fail.

Fixes: 345e0d35ec ("arm64: introduce aarch64_insn_gen_cond_branch_imm()")
Fixes: c94ae4f7c5 ("arm64: insn: remove BUG_ON from codegen")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20260415121403.639619-1-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

arch/arm64/lib/insn.c

@@ -338,6 +338,8 @@ u32 aarch64_insn_gen_cond_branch_imm(unsigned long pc, unsigned long addr,
 	long offset;
 
 	offset = label_imm_common(pc, addr, SZ_1M);
+	if (offset >= SZ_1M)
+		return AARCH64_BREAK_FAULT;
 
 	insn = aarch64_insn_get_bcond_value();