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

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Message-ID: <CAADnVQKw5x6sTwj62p4vxSqtjdisHEKhtKdPp_zK4t7rtDuWhQ@mail.gmail.com>
Date: Tue, 13 Aug 2024 11:57:00 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Thorsten Blum <thorsten.blum@...lux.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 Tue, Aug 13, 2024 at 10:59 AM Thorsten Blum <thorsten.blum@...lux.com> wrote:
>
> 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.

What kind of error/warn do you see ?
Is it runtime or compile time?

Is this the only place?
what about:
        new_cands = kmemdup(cands, sizeof_cands(cands->cnt), GFP_KERNEL);

cnt field gets copied with other fields.
Can compiler/runtime catch that?

> 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/