linux-kernel - Re: [PATCH] bpf: Annotate struct bpf_cand_cache with __counted

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Message-Id: <2A7DB1E6-4CCE-446E-B6F1-4A99D3F87B57@toblux.com>
Date: Tue, 13 Aug 2024 19:59:25 +0200
From: Thorsten Blum <thorsten.blum@...lux.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Martin KaFai Lau <martin.lau@...ux.dev>,
 Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>,
 Andrii Nakryiko <andrii@...nel.org>,
 Eddy Z <eddyz87@...il.com>,
 Song Liu <song@...nel.org>,
 Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>,
 KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>,
 Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>,
 Kees Cook <kees@...nel.org>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>,
 bpf <bpf@...r.kernel.org>,
 LKML <linux-kernel@...r.kernel.org>,
 linux-hardening@...r.kernel.org
Subject: Re: [PATCH] bpf: Annotate struct bpf_cand_cache with __counted_by()

On 13. Aug 2024, at 18:28, Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> On Tue, Aug 13, 2024 at 8:19 AM Thorsten Blum <thorsten.blum@...lux.com> wrote:
>> 
>> Add the __counted_by compiler attribute to the flexible array member
>> cands to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
>> CONFIG_FORTIFY_SOURCE.
>> 
>> Increment cnt before adding a new struct to the cands array.
> 
> why? What happens otherwise?

If you try to access cands->cands[cands->cnt] without incrementing
cands->cnt first, you're essentially accessing the array out of bounds
which will fail during runtime.

You can read more about it at [1] and [2].

> Signed-off-by: Thorsten Blum <thorsten.blum@...lux.com>
>> ---
>> kernel/bpf/btf.c | 6 +++---
>> 1 file changed, 3 insertions(+), 3 deletions(-)
>> 
>> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
>> index 520f49f422fe..42bc70a56fcd 100644
>> --- a/kernel/bpf/btf.c
>> +++ b/kernel/bpf/btf.c
>> @@ -7240,7 +7240,7 @@ struct bpf_cand_cache {
>>        struct {
>>                const struct btf *btf;
>>                u32 id;
>> -       } cands[];
>> +       } cands[] __counted_by(cnt);
>> };
>> 
>> static DEFINE_MUTEX(cand_cache_mutex);
>> @@ -8784,9 +8784,9 @@ bpf_core_add_cands(struct bpf_cand_cache *cands, const struct btf *targ_btf,
>>                memcpy(new_cands, cands, sizeof_cands(cands->cnt));
>>                bpf_free_cands(cands);
>>                cands = new_cands;
>> -               cands->cands[cands->cnt].btf = targ_btf;
>> -               cands->cands[cands->cnt].id = i;
>>                cands->cnt++;
>> +               cands->cands[cands->cnt - 1].btf = targ_btf;
>> +               cands->cands[cands->cnt - 1].id = i;
>>        }
>>        return cands;
>> }
>> --
>> 2.46.0
>> 

[1] https://opensource.googleblog.com/2024/07/bounds-checking-flexible-array-members.html
[2] https://embeddedor.com/blog/2024/06/18/how-to-use-the-new-counted_by-attribute-in-c-and-linux/