bpf: Reject kfunc calls that overflow insn->imm

Now kfunc call uses s32 to represent the offset between the address of
kfunc and __bpf_call_base, but it doesn't check whether or not s32 will
be overflowed. The overflow is possible when kfunc is in module and the
offset between module and kernel is greater than 2GB. Take arm64 as an
example, before commit b2eed9b5 ("arm64/kernel: kaslr: reduce module
randomization range to 2 GB"), the offset between module symbol and
__bpf_call_base will in 4GB range due to KASLR and may overflow s32.

So add an extra checking to reject these invalid kfunc calls.
Signed-off-by: NHou Tao <houtao1@huawei.com>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>
Acked-by: NYonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220215065732.3179408-1-houtao1@huawei.com

kernel/bpf/verifier.c

@@ -1842,6 +1842,7 @@ static int add_kfunc_call(struct bpf_verifier_env *env, u32 func_id, s16 offset)
 	struct bpf_kfunc_desc *desc;
 	const char *func_name;
 	struct btf *desc_btf;
+	unsigned long call_imm;
 	unsigned long addr;
 	int err;
 
@@ -1926,9 +1927,17 @@ static int add_kfunc_call(struct bpf_verifier_env *env, u32 func_id, s16 offset)
 		return -EINVAL;
 	}
 
+	call_imm = BPF_CALL_IMM(addr);
+	/* Check whether or not the relative offset overflows desc->imm */
+	if ((unsigned long)(s32)call_imm != call_imm) {
+		verbose(env, "address of kernel function %s is out of range\n",
+			func_name);
+		return -EINVAL;
+	}
+
 	desc = &tab->descs[tab->nr_descs++];
 	desc->func_id = func_id;
-	desc->imm = BPF_CALL_IMM(addr);
+	desc->imm = call_imm;
 	desc->offset = offset;
 	err = btf_distill_func_proto(&env->log, desc_btf,
 				     func_proto, func_name,