Re: [PATCH bpf-next v5 5/6] libbpf: Add log_opts support for bpf_link_create

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] libbpf: type confusion between iter_info and log_opts in bpf_link_create()
--

commit b4af2771f4c0edb6656b6ddcd881019158c55a55
Author: Leon Hwang <leon.hwang@linux.dev>

libbpf: Add log_opts support for bpf_link_create

This commit adds log_opts support to bpf_link_create() by placing log_opts in
an anonymous union with iter_info in struct bpf_link_create_opts and passing
it to __sys_bpf_fd().

> diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
> index aea534764dfe2..38dcb31e8c0ed 100644
> --- a/tools/lib/bpf/bpf.c
> +++ b/tools/lib/bpf/bpf.c
> @@ -794,6 +794,8 @@ int bpf_link_create(int prog_fd, int target_fd,
>  {
>  	const size_t attr_sz = offsetofend(union bpf_attr, link_create);
>  	__u32 target_btf_id, iter_info_len, relative_id;
> +	union bpf_iter_link_info *iter_info = NULL;
> +	struct bpf_log_opts *log_opts = NULL;
>  	int fd, err, relative_fd;
>  	union bpf_attr attr;
>  
> @@ -803,6 +805,14 @@ int bpf_link_create(int prog_fd, int target_fd,
>  	iter_info_len = OPTS_GET(opts, iter_info_len, 0);
>  	target_btf_id = OPTS_GET(opts, target_btf_id, 0);
>  
> +	if (iter_info_len && attach_type != BPF_TRACE_ITER)
> +		return libbpf_err(-EINVAL);
> +
> +	if (iter_info_len)
> +		iter_info = OPTS_GET(opts, iter_info, (void *)0);
> +	else
> +		log_opts = OPTS_GET(opts, log_opts, (void *)0);

[Severity: Medium]
Can overlaying iter_info and log_opts in struct bpf_link_create_opts and
selecting between them using only iter_info_len cause type confusion when a
caller passes a non-NULL iter_info with iter_info_len == 0?

For example, a caller of bpf_link_create() or bpf_program__attach_iter() in
tools/lib/bpf/libbpf.c:

tools/lib/bpf/libbpf.c:bpf_program__attach_iter() {
	...
	link_create_opts.iter_info = OPTS_GET(opts, link_info, (void *)0);
	link_create_opts.iter_info_len = OPTS_GET(opts, link_info_len, 0);
	...
	return bpf_link_create(prog_fd, target_fd, BPF_TRACE_ITER, &link_create_opts);
}

might pass a non-NULL union bpf_iter_link_info pointer while leaving
iter_info_len (or link_info_len) set to 0.

Previously, bpf_link_create() forwarded both the non-NULL iter_info pointer
and iter_info_len == 0 to the kernel, where bpf_iter_link_attach() rejected
the mismatched arguments with -EINVAL:

kernel/bpf/bpf_iter.c:bpf_iter_link_attach() {
	...
	if (bpfptr_is_null(ulinfo) ^ !linfo_len)
		return -EINVAL;
	...
}

With this change, when iter_info_len == 0, bpf_link_create() leaves iter_info
as NULL (so attr.link_create.iter_info is 0 and the kernel check is bypassed)
and instead assigns the union bpf_iter_link_info pointer to log_opts.

When bpf_link_create() then calls __sys_bpf_fd(), log_opts is dereferenced as
struct bpf_log_opts:

tools/lib/bpf/bpf.c:__sys_bpf_fd() {
	...
	if (!OPTS_VALID(log_opts, bpf_log_opts))
		return libbpf_err(-EINVAL);

	if (log_opts && feat_supported(NULL, FEAT_BPF_SYSCALL_COMMON_ATTRS)) {
		memset(&attr_common, 0, attr_common_sz);
		attr_common.log_buf = ptr_to_u64(OPTS_GET(log_opts, buf, NULL));
		attr_common.log_size = OPTS_GET(log_opts, size, 0);
		attr_common.log_level = OPTS_GET(log_opts, level, 0);
		fd = sys_bpf_ext_fd(cmd, attr, attr_sz, &attr_common, attr_common_sz);
		OPTS_SET(log_opts, true_size, attr_common.log_true_size);
	} else {
		fd = sys_bpf_fd(cmd, attr, attr_sz);
		OPTS_SET(log_opts, true_size, 0);
	}
	...
}

Since union bpf_iter_link_info is 16 bytes while struct bpf_log_opts extends
to offsetofend(struct bpf_log_opts, true_size) (28 bytes on 64-bit, 20 bytes
on 32-bit), interpreting the first sizeof(size_t) bytes of iter_info (such as
map.map_fd, task.tid, task.pid, or cgroup.cgroup_fd) as log_opts->sz can
cause out-of-bounds reads in OPTS_VALID() and OPTS_GET(), as well as a 4-byte
out-of-bounds write past union bpf_iter_link_info via OPTS_SET(log_opts,
true_size, ...).

In addition, aliasing iter_info and log_opts prevents BPF_TRACE_ITER links
from using log_opts whenever iter_info is specified. Would it be safer to
place log_opts in its own field at the end of struct bpf_link_create_opts
instead of sharing a union with iter_info?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261026154948.99408-1-leon.hwang@linux.dev?part=5