netdev - Re: [PATCH bpf-next v2] bpf: reject kfunc calls that overflow insn->imm

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Date:   Tue, 8 Feb 2022 08:57:30 -0800
From:   Yonghong Song <yhs@...com>
To:     Hou Tao <houtao1@...wei.com>, Alexei Starovoitov <ast@...nel.org>
CC:     Martin KaFai Lau <kafai@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Song Liu <songliubraving@...com>,
        John Fastabend <john.fastabend@...il.com>,
        <netdev@...r.kernel.org>, <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2] bpf: reject kfunc calls that overflow
 insn->imm



On 2/8/22 4:33 AM, Hou Tao wrote:
> 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, so add an extra checking to reject these
> invalid kfunc calls.
> 
> Signed-off-by: Hou Tao <houtao1@...wei.com>
> ---
> v2:
>   * instead of checking the overflow in selftests, just reject
>     these kfunc calls directly in verifier
> 
> v1: https://lore.kernel.org/bpf/20220206043107.18549-1-houtao1@huawei.com
> ---
>   kernel/bpf/verifier.c | 13 +++++++++++++
>   1 file changed, 13 insertions(+)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index a39eedecc93a..fd836e64b701 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -1832,6 +1832,13 @@ static struct btf *find_kfunc_desc_btf(struct bpf_verifier_env *env,
>   	return btf_vmlinux ?: ERR_PTR(-ENOENT);
>   }
>   
> +static inline bool is_kfunc_call_imm_overflowed(unsigned long addr)
> +{
> +	unsigned long offset = BPF_CALL_IMM(addr);
> +
> +	return (unsigned long)(s32)offset != offset;
> +}
> +
>   static int add_kfunc_call(struct bpf_verifier_env *env, u32 func_id, s16 offset)
>   {
>   	const struct btf_type *func, *func_proto;
> @@ -1925,6 +1932,12 @@ static int add_kfunc_call(struct bpf_verifier_env *env, u32 func_id, s16 offset)
>   		return -EINVAL;
>   	}
>   
> +	if (is_kfunc_call_imm_overflowed(addr)) {
> +		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);

Thanks, I would like to call BPF_CALL_IMM only once and keep checking 
overflow and setting desc->imm close to each other. How about the 
following not-compile-tested code

	unsigned long call_imm;

	...
	call_imm = BPF_CALL_IMM(addr);
	/* some comment here */
	if ((unsigned long)(s32)call_imm != call_imm) {
		verbose(env, ...);
		return -EINVAL;
	} else {
		desc->imm = call_imm;
	}