netdev - Re: [PATCH v4 net-next 12/12] mlx5: support BIG TCP packets

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Date:   Sat, 7 May 2022 04:19:06 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>,
        netdev <netdev@...r.kernel.org>, Coco Li <lixiaoyan@...gle.com>,
        Tariq Toukan <tariqt@...dia.com>,
        Saeed Mahameed <saeedm@...dia.com>,
        Leon Romanovsky <leon@...nel.org>
Subject: Re: [PATCH v4 net-next 12/12] mlx5: support BIG TCP packets

On Sat, May 7, 2022 at 12:46 AM Kees Cook <keescook@...omium.org> wrote:
>
> On Fri, May 06, 2022 at 06:54:05PM -0700, Jakub Kicinski wrote:
> > On Fri, 6 May 2022 17:32:43 -0700 Eric Dumazet wrote:
> > > On Fri, May 6, 2022 at 3:34 PM Jakub Kicinski <kuba@...nel.org> wrote:
> > > > In function ‘fortify_memcpy_chk’,
> > > >     inlined from ‘mlx5e_sq_xmit_wqe’ at ../drivers/net/ethernet/mellanox/mlx5/core/en_tx.c:408:5:
> > > > ../include/linux/fortify-string.h:328:25: warning: call to ‘__write_overflow_field’ declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Wattribute-warning]
> > > >   328 |                         __write_overflow_field(p_size_field, size);
> > > >       |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> Ah, my old friend, inline_hdr.start. Looks a lot like another one I fixed
> earlier in ad5185735f7d ("net/mlx5e: Avoid field-overflowing memcpy()"):
>
>         if (attr->ihs) {
>                 if (skb_vlan_tag_present(skb)) {
>                         eseg->inline_hdr.sz |= cpu_to_be16(attr->ihs + VLAN_HLEN);
>                         mlx5e_insert_vlan(eseg->inline_hdr.start, skb, attr->ihs);
>                         stats->added_vlan_packets++;
>                 } else {
>                         eseg->inline_hdr.sz |= cpu_to_be16(attr->ihs);
>                         memcpy(eseg->inline_hdr.start, skb->data, attr->ihs);
>                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>                 }
>                 dseg += wqe_attr->ds_cnt_inl;
>
> This is actually two regions, 2 bytes in eseg and everything else in
> dseg. Splitting the memcpy() will work:
>
>         memcpy(eseg->inline_hdr.start, skb->data, sizeof(eseg->inline_hdr.start));
>         memcpy(dseg, skb->data + sizeof(eseg->inline_hdr.start), ihs - sizeof(eseg->inline_hdr.start));
>
> But this begs the question, what is validating that ihs -2 is equal to
> wqe_attr->ds_cnt_inl * sizeof(*desg) ?
>
> And how is wqe bounds checked?

Look at the definition of struct mlx5i_tx_wqe

Then mlx5i_sq_calc_wqe_attr() computes the number of ds_cnt  (16 bytes
granularity)
units needed.

Then look at mlx5e_txqsq_get_next_pi()

I doubt a compiler can infer that the driver is correct.

Basically this is variable length structure, quite common in NIC
world, given number of dma descriptor can vary from 1 to XX,
and variable size of headers. (Typically, fast NIC want to get the
headers inlined in TX descriptor)


>
>
> > > > In function ‘fortify_memcpy_chk’,
> > > >     inlined from ‘mlx5i_sq_xmit’ at ../drivers/net/ethernet/mellanox/mlx5/core/en_tx.c:962:4:
> > > > ../include/linux/fortify-string.h:328:25: warning: call to ‘__write_overflow_field’ declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Wattribute-warning]
> > > >   328 |                         __write_overflow_field(p_size_field, size);
> > > >       |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> And moar inline_hdr.start:
>
>         if (attr.ihs) {
>                 memcpy(eseg->inline_hdr.start, skb->data, attr.ihs);
>                 eseg->inline_hdr.sz = cpu_to_be16(attr.ihs);
>                 dseg += wqe_attr.ds_cnt_inl;
>         }
>
> again, a split:
>
>         memcpy(eseg->inline_hdr.start, skb->data, sizeof(eseg->inline_hdr.start));
>         eseg->inline_hdr.sz = cpu_to_be16(attr.ihs);
>         memcpy(dseg, skb->data + sizeof(eseg->inline_hdr.start), ihs - sizeof(eseg->inline_hdr.start));
>         dseg += wqe_attr.ds_cnt_inl;
>
> And the same bounds questions come up.
>
> It'd be really nice to get some kind of generalized "copy out of
> skb->data with bounds checking that may likely all get reduced to
> constant checks".


NIC drivers send millions of packets per second.
We can not really afford copying each component of a frame one byte at a time.

The memcpy() here can typically copy IPv6 header (40 bytes) + TCP
header (up to 60 bytes), plus more headers if encapsulation is added.

Thanks.