Re: [PATCH bpf-next] bpf: ringbuf: Support consuming BPF_MAP_TYPE_RINGBUF from prog

[Daniel Xu]

On Tue, Sep 10, 2024 at 03:21:04PM GMT, Andrii Nakryiko wrote:
> On Tue, Sep 10, 2024 at 3:16 PM Daniel Xu <dxu@xxxxxxxxx> 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.

Ah, ok. I think I understand.

Given this and your other comments about rb->busy, what about enforcing
bpf_user_ringbuf_drain() NAND mmap? I think the use cases here are
different enough where this makes sense.