On Tue, Aug 17, 2021 at 10:36 AM Colin Ian King
<colin.king@xxxxxxxxxxxxx> wrote:
>
> Hi,
>
> Static analysis with Coverity on linux-next has detected a potential
> issue with the following commit:
>
> commit b89fbfbb854c9afc3047e8273cc3a694650b802e
> Author: Andrii Nakryiko <andrii@xxxxxxxxxx>
> Date: Sun Aug 15 00:05:57 2021 -0700
>
> bpf: Implement minimal BPF perf link
>
> The analysis is as follows:
>
> 2936 static int bpf_perf_link_attach(const union bpf_attr *attr, struct
> bpf_prog *prog)
> 2937 {
>
> 1. var_decl: Declaring variable link_primer without initializer.
>
> 2938 struct bpf_link_primer link_primer;
> 2939 struct bpf_perf_link *link;
> 2940 struct perf_event *event;
> 2941 struct file *perf_file;
> 2942 int err;
> 2943
>
> 2. Condition attr->link_create.flags, taking false branch.
>
> 2944 if (attr->link_create.flags)
> 2945 return -EINVAL;
> 2946
> 2947 perf_file = perf_event_get(attr->link_create.target_fd);
>
> 3. Condition IS_ERR(perf_file), taking false branch.
>
> 2948 if (IS_ERR(perf_file))
> 2949 return PTR_ERR(perf_file);
> 2950
> 2951 link = kzalloc(sizeof(*link), GFP_USER);
>
> 4. Condition !link, taking false branch.
>
> 2952 if (!link) {
> 2953 err = -ENOMEM;
> 2954 goto out_put_file;
> 2955 }
> 2956 bpf_link_init(&link->link, BPF_LINK_TYPE_PERF_EVENT,
> &bpf_perf_link_lops, prog);
> 2957 link->perf_file = perf_file;
> 2958
> 2959 err = bpf_link_prime(&link->link, &link_primer);
>
> 5. Condition err, taking false branch.
>
> 2960 if (err) {
> 2961 kfree(link);
> 2962 goto out_put_file;
> 2963 }
> 2964
> 2965 event = perf_file->private_data;
> 2966 err = perf_event_set_bpf_prog(event, prog,
> attr->link_create.perf_event.bpf_cookie);
>
> 6. Condition err, taking true branch.
> 2967 if (err) {
> 7. uninit_use_in_call: Using uninitialized value link_primer.fd when
> calling bpf_link_cleanup.
> 8. uninit_use_in_call: Using uninitialized value link_primer.file
> when calling bpf_link_cleanup.
> 9. uninit_use_in_call: Using uninitialized value link_primer.id when
> calling bpf_link_cleanup.
>
> Uninitialized pointer read (UNINIT)
> 10. uninit_use_in_call: Using uninitialized value link_primer.link
> when calling bpf_link_cleanup.
>
> 2968 bpf_link_cleanup(&link_primer);
> 2969 goto out_put_file;
> 2970 }
> 2971 /* perf_event_set_bpf_prog() doesn't take its own refcnt on
> prog */
> 2972 bpf_prog_inc(prog);
>
> I'm not 100% sure if these are false-positives, but I thought I should
> report the issues as potentially there is a pointer access on an
> uninitialized pointer on line 2968.
Look at bpf_link_prime() implementation. If it succeeds, link_primer
is fully initialized. We use this pattern in many places, this is the
first time someone reports any potential issues with it. It's a bit
strange that Coverity doesn't recognize such a typical output
parameter initialization pattern, tbh. Maybe the global nature of
bpf_link_prime() throws it off (it assumes it can be "overridden"
during linking?) But I double-checked everything twice, all seems to
be good. Zero-initializing link_primer would be a total waste.
>
> Colin
