linux-kernel - Re: [PATCH bpf-next] bpf: ringbuf: Support consuming BPF_MAP_TYPE

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Message-ID: <CAEf4BzbHqKD87KTSmFUMokXEaAa70xNs96QqfWBHjFbuE5PL=w@mail.gmail.com>
Date: Tue, 10 Sep 2024 15:21:04 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Daniel Xu <dxu@...uu.xyz>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>, Eduard Zingerman <eddyz87@...il.com>, 
	Andrii Nakryiko <andrii@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Alexei Starovoitov <ast@...nel.org>, Shuah Khan <shuah@...nel.org>, 
	John Fastabend <john.fastabend@...il.com>, Martin KaFai Lau <martin.lau@...ux.dev>, 
	Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	Mykola Lysenko <mykolal@...com>, LKML <linux-kernel@...r.kernel.org>, 
	"bpf@...r.kernel.org" <bpf@...r.kernel.org>, 
	"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>, Kernel Team <kernel-team@...a.com>
Subject: Re: [PATCH bpf-next] bpf: ringbuf: Support consuming
 BPF_MAP_TYPE_RINGBUF from prog

On Tue, Sep 10, 2024 at 3:16 PM Daniel Xu <dxu@...uu.xyz> wrote:
>
>
>
> On Tue, Sep 10, 2024, at 2:07 PM, Daniel Xu wrote:
> > On Tue, Sep 10, 2024 at 01:41:41PM GMT, Andrii Nakryiko wrote:
> >> On Tue, Sep 10, 2024 at 11:36 AM Alexei Starovoitov
> [...]
> >
> >>
> >> Also, Daniel, can you please make sure that dynptr we return for each
> >> sample is read-only? We shouldn't let consumer BPF program ability to
> >> corrupt ringbuf record headers (accidentally or otherwise).
> >
> > Sure.
>
> So the sample is not read-only. But I think prog is prevented from messing
> with header regardless.
>
> __bpf_user_ringbuf_peek() returns sample past the header:
>
>         *sample = (void *)((uintptr_t)rb->data +
>                            (uintptr_t)((cons_pos + BPF_RINGBUF_HDR_SZ) & rb->mask));
>
> dynptr is initialized with the above ptr:
>
>         bpf_dynptr_init(&dynptr, sample, BPF_DYNPTR_TYPE_LOCAL, 0, size);
>
> So I don't think there's a way for the prog to access the header thru the dynptr.
>

By "header" I mean 8 bytes that precede each submitted ringbuf record.
That header is part of ringbuf data area. Given user space can set
consumer_pos to arbitrary value, kernel can return arbitrary part of
ringbuf data area, including that 8 byte header. If that data is
writable, it's easy to screw up that header and crash another BPF
program that reserves/submits a new record. User space can only read
data area for BPF ringbuf, and so we rely heavily on a tight control
of who can write what into those 8 bytes.

> Thanks,
> Daniel