linux-kernel - Re: [PATCH bpf-next v5 2/2] bpf: fix stackmap overflow check in __bpf_get

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Message-ID: <CAADnVQ+6bV3h3i-A1LHbEk=nY_PMx69BiogWjf5GtGaLxWSQVg@mail.gmail.com>
Date: Fri, 29 Aug 2025 10:29:20 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Arnaud Lecomte <contact@...aud-lcm.com>
Cc: Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Andrii Nakryiko <andrii@...nel.org>, 
	Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Eduard <eddyz87@...il.com>, Hao Luo <haoluo@...gle.com>, 
	John Fastabend <john.fastabend@...il.com>, Jiri Olsa <jolsa@...nel.org>, 
	KP Singh <kpsingh@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Stanislav Fomichev <sdf@...ichev.me>, syzbot+c9b724fbb41cf2538b7b@...kaller.appspotmail.com, 
	syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: [PATCH bpf-next v5 2/2] bpf: fix stackmap overflow check in __bpf_get_stackid()

On Tue, Aug 26, 2025 at 2:24 PM Arnaud Lecomte <contact@...aud-lcm.com> wrote:
>
> Syzkaller reported a KASAN slab-out-of-bounds write in __bpf_get_stackid()
> when copying stack trace data. The issue occurs when the perf trace
>  contains more stack entries than the stack map bucket can hold,
>  leading to an out-of-bounds write in the bucket's data array.
>
> Changes in v2:
>  - Fixed max_depth names across get stack id
>
> Changes in v4:
>  - Removed unnecessary empty line in __bpf_get_stackid
>
> Link to v4: https://lore.kernel.org/all/20250813205506.168069-1-contact@arnaud-lcm.com/
>
> Reported-by: syzbot+c9b724fbb41cf2538b7b@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=c9b724fbb41cf2538b7b
> Signed-off-by: Arnaud Lecomte <contact@...aud-lcm.com>
> Acked-by: Yonghong Song <yonghong.song@...ux.dev>
> ---
>  kernel/bpf/stackmap.c | 23 +++++++++++++----------
>  1 file changed, 13 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index 796cc105eacb..ef8269ab8d6f 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -247,7 +247,7 @@ get_callchain_entry_for_task(struct task_struct *task, u32 max_depth)
>  }
>
>  static long __bpf_get_stackid(struct bpf_map *map,
> -                             struct perf_callchain_entry *trace, u64 flags)
> +                             struct perf_callchain_entry *trace, u64 flags, u32 max_depth)
>  {
>         struct bpf_stack_map *smap = container_of(map, struct bpf_stack_map, map);
>         struct stack_map_bucket *bucket, *new_bucket, *old_bucket;
> @@ -263,6 +263,8 @@ static long __bpf_get_stackid(struct bpf_map *map,
>
>         trace_nr = trace->nr - skip;
>         trace_len = trace_nr * sizeof(u64);
> +       trace_nr = min(trace_nr, max_depth - skip);
> +

The patch might have fixed this particular syzbot repro
with OOB in stackmap-with-buildid case,
but above two line looks wrong.
trace_len is computed before being capped by max_depth.
So non-buildid case below is using
memcpy(new_bucket->data, ips, trace_len);

so OOB is still there?