netdev - Re: [PATCH bpf-next v2 4/8] libbpf: Ensure that BPF syscall fds are never 0, 1, or 2

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Date:   Thu, 14 Oct 2021 16:55:54 +0000
From:   Song Liu <songliubraving@...com>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
CC:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        "Martin Lau" <kafai@...com>, Yonghong Song <yhs@...com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 4/8] libbpf: Ensure that BPF syscall fds are
 never 0, 1, or 2



> On Oct 13, 2021, at 12:33 AM, Kumar Kartikeya Dwivedi <memxor@...il.com> wrote:
> 
> Add a simple wrapper for passing an fd and getting a new one >= 3 if it
> is one of 0, 1, or 2. There are two primary reasons to make this change:
> First, libbpf relies on the assumption a certain BPF fd is never 0 (e.g.
> most recently noticed in [0]). Second, Alexei pointed out in [1] that
> some environments reset stdin, stdout, and stderr if they notice an
> invalid fd at these numbers. To protect against both these cases, switch
> all internal BPF syscall wrappers in libbpf to always return an fd >= 3.
> We only need to modify the syscall wrappers and not other code that
> assumes a valid fd by doing >= 0, to avoid pointless churn, and because
> it is still a valid assumption. The cost paid is two additional syscalls
> if fd is in range [0, 2].

Acked-by: Song Liu <songliubraving@...com>

With on nitpick below. 

[...]

> +static inline int skel_reserve_bad_fds(struct bpf_load_and_run_opts *opts, int *fds)
> +{
> +	int fd, err, i;
> +
> +	for (i = 0; i < 3; i++) {
> +		fd = open("/dev/null", O_RDONLY | O_CLOEXEC);
> +		if (fd < 0) {
> +			opts->errstr = "failed to reserve fd 0, 1, and 2";
> +			err = -errno;
> +			return err;
> +		}
> +		if (fd >= 3)
> +			close(fd);

nit: Maybe likely(fd >= 3) and break here? 

> +		else
> +			fds[i] = fd;
> +	}
> +	return 0;
> +}
> +
> static inline int bpf_load_and_run(struct bpf_load_and_run_opts *opts)
> {
> -	int map_fd = -1, prog_fd = -1, key = 0, err;
> +	int map_fd = -1, prog_fd = -1, key = 0, err, i;
> +	int res_fds[3] = { -1, -1, -1 };
> 	union bpf_attr attr;
> 
> +	/* ensures that we don't open fd 0, 1, or 2 from here on out */
> +	err = skel_reserve_bad_fds(opts, res_fds);
> +	if (err < 0) {
> +		errno = -err;
> +		goto out;
> +	}
> +
> 	map_fd = bpf_create_map_name(BPF_MAP_TYPE_ARRAY, "__loader.map", 4,
> 				     opts->data_sz, 1, 0);
> 	if (map_fd < 0) {
> @@ -115,6 +143,10 @@ static inline int bpf_load_and_run(struct bpf_load_and_run_opts *opts)
> 	}
> 	err = 0;
> out:
> +	for (i = 0; i < 3; i++) {
> +		if (res_fds[i] >= 0)
> +			close(res_fds[i]);
> +	}
> 	if (map_fd >= 0)
> 		close(map_fd);
> 	if (prog_fd >= 0)
> -- 
> 2.33.0
>