netdev - Re: [bpf PATCH 4/9] bpf: sockmap, skmsg helper overestimates push, pull, and pop bounds

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Date:   Thu, 9 Jan 2020 10:37:28 -0800
From:   Song Liu <song@...nel.org>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [bpf PATCH 4/9] bpf: sockmap, skmsg helper overestimates push,
 pull, and pop bounds

On Wed, Jan 8, 2020 at 1:15 PM John Fastabend <john.fastabend@...il.com> wrote:
>
> In the push, pull, and pop helpers operating on skmsg objects to make
> data writable or insert/remove data we use this bounds check to ensure
> specified data is valid,
>
>  /* Bounds checks: start and pop must be inside message */
>  if (start >= offset + l || last >= msg->sg.size)
>      return -EINVAL;
>
> The problem here is offset has already included the length of the
> current element the 'l' above. So start could be past the end of
> the scatterlist element in the case where start also points into an
> offset on the last skmsg element.
>
> To fix do the accounting slightly different by adding the length of
> the previous entry to offset at the start of the iteration. And
> ensure its initialized to zero so that the first iteration does
> nothing.
>
> Fixes: 604326b41a6fb ("bpf, sockmap: convert to generic sk_msg interface")
> Fixes: 6fff607e2f14b ("bpf: sk_msg program helper bpf_msg_push_data")
> Fixes: 7246d8ed4dcce ("bpf: helper to pop data from messages")
> Signed-off-by: John Fastabend <john.fastabend@...il.com>

This is pretty tricky... But it looks right.

Acked-by: Song Liu <songliubraving@...com>