Re: [PATCH net-next v6 7/8] net: check for driver support in netmem TX

[Mina Almasry <almasrymina@google.com>]

On Mon, Mar 3, 2025 at 4:29 PM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Fri, 28 Feb 2025 17:53:24 -0800 Mina Almasry wrote:
> > On Fri, Feb 28, 2025 at 4:43 PM Jakub Kicinski <kuba@kernel.org> wrote:
> > > On Thu, 27 Feb 2025 04:12:08 +0000 Mina Almasry wrote:
> > > > +     if (!skb_frags_readable(skb) && !dev->netmem_tx)
> > >
> > > How do you know it's for _this_ device tho?
> >
> > Maybe a noob question, but how do we end up here with an skb that is
> > not targeted for the 'dev' device? We are checking in
> > tcp_sendmsg_locked that we're targeting the appropriate device before
> > creating the skb. Is this about a packet arriving on a dmabuf bound to
> > a device and then being forwarded through another device that doesn't
> > own the mapping, bypassing the check?
>
> Forwarded or just redirected by nft/bpf/tc
>
> > > The driver doesn't seem to check the DMA mapping belongs to it either.
> > >
> > > Remind me, how do we prevent the unreadable skbs from getting into the
> > > Tx path today?
> >
> > I'm not sure if this is about forwarding, or if there is some other
> > way for unreadable skbs to end up in the XT path that you have in
> > mind. At some point in this thread[1] we had talked about preventing
> > MP bound devices from being lower devices at all to side step this
> > entirely but you mentioned that may not be enough, and we ended up
> > sidestepping only XDP entirely.
> >
> > [1] https://lore.kernel.org/bpf/20240821153049.7dc983db@kernel.org/
>
> Upper devices and BPF access is covered I think, by the skbuff checks.
> But I think we missed adding a check in validate_xmit_skb() to protect
> the xmit paths of HW|virt drivers. You can try to add a TC rule which
> forwards all traffic from your devmem flow back out to the device and
> see if it crashes on net-next ?

No crash, but by adding debug logs I'm detecting that we're passing
unreadable netmem dma-addresses to the dma_unmap_*() APIs, which is
known to be unsafe. I just can't reproduce an issue because my
platform has the IOMMU disabled.

I guess I do need to send the hunk from validate_xmit_skb() as a fix
to net and CC stable.

Another thing I'm worried about is ip_forward() inserting an
unreadable skb into the tx path somewhere higher up the stack which
calls more code that isn't expecting unreadable skbs? Specifically
worried about skb_frag_ref/unref. Does this sound like a concern as
well? Or is it a similar code path to tc?

--
Thanks,
Mina