Re: [PATCH net 2/5] eth: bnxt: resize Rx head buffers when rx-copybreak changes

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- [High] The fix is incomplete.

Pre-existing issues:
- [Medium] This problem wasn't introduced by this patch, but
  bnxt_copy_data() (drivers/net/ethernet/broadcom/bnxt/bnxt.c) syncs…

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eth: bnxt: resize Rx head buffers when rx-copybreak changes

This patch makes bnxt_set_tunable() call bnxt_set_ring_params() after it
changes bp->rx_copybreak, so the Rx head buffers are resized before the
next open. The goal is to stop bnxt_copy_data() from syncing rx_copybreak
bytes past the end of head buffers that are too small.

[Severity: High]

Does this cover the case where no aggregation ring is in use?

The commit message says bnxt_set_ring_params() sizes the head buffers to
fit at least rx_copybreak bytes. The rx_copybreak floor only exists inside
the agg_factor branch, though:

drivers/net/ethernet/broadcom/bnxt/bnxt.c:bnxt_set_ring_params() {
    ...
	if (agg_factor) {
		...
		if (BNXT_RX_PAGE_MODE(bp)) {
			...
		} else {
			rx_size = max3(BNXT_MIN_RX_HDR_BUF,
				       bp->rx_copybreak,
				       bp->dev->cfg_pending->hds_thresh);
			...
		}
	}

	bp->rx_buf_use_size = rx_size;
	bp->rx_buf_size = rx_space;
    ...
}

agg_factor is 0 when TPA is off, HDS is off and rx_space <= PAGE_SIZE.
That happens after ethtool -K gro-hw/lro off, in a kdump kernel, or with
BNXT_FLAG_NO_AGG_RINGS. In that case the sizes come only from the MTU:

	rx_size = SKB_DATA_ALIGN(bp->dev->mtu + ETH_HLEN + NET_IP_ALIGN + 8);

	rx_space = rx_size + ALIGN(max(NET_SKB_PAD, XDP_PACKET_HEADROOM), 8) +
		SKB_DATA_ALIGN(sizeof(struct skb_shared_info));

bnxt_set_tunable() still accepts any rx_copybreak up to
BNXT_MAX_RX_HDR_BUF. bnxt_copy_data() still syncs a fixed
bp->rx_copybreak bytes:

	dma_sync_single_for_cpu(&pdev->dev, mapping, bp->rx_copybreak,
				bp->rx_dir);
	...
	dma_sync_single_for_device(&pdev->dev, mapping, bp->rx_copybreak,
				   bp->rx_dir);

__bnxt_alloc_rx_frag() packs rx_buf_size fragments into one head_pool
page, and each mapping starts NET_SKB_PAD bytes into its fragment. Two
examples on x86-64 with rx-copybreak set to 1024:

- MTU 60: rx_buf_size is 704, so each sync runs 384 bytes into the next
  fragment.
- MTU 400: rx_buf_size is 1024, with four fragments per page. The sync
  for the last fragment ends 64 bytes past the page and outside its DMA
  mapping.

The BD length here is rx_buf_use_size, which is <= rx_copybreak, so every
received packet goes through bnxt_rx_pkt()->bnxt_copy_skb()->
bnxt_copy_data().

Isn't the oversized sync from the commit message still reachable on this
path? With swiotlb or non-coherent DMA it could still corrupt neighbouring
buffers. The later patches in the series don't seem to touch
bnxt_set_ring_params() or the sync length in bnxt_copy_data().

Would any of these be a more complete fix?

- bound the sync to len + NET_IP_ALIGN
- apply the rx_copybreak floor on the non-agg path as well
- reject rx_copybreak values larger than the current head buffer

[Severity: Medium]

This is a pre-existing issue, not introduced by this patch. In
bnxt_copy_data() the CPU sync looks shorter than the memcpy that follows
it:

	dma_sync_single_for_cpu(&pdev->dev, mapping, bp->rx_copybreak,
				bp->rx_dir);

	memcpy(skb->data - NET_IP_ALIGN, data - NET_IP_ALIGN,
	       len + NET_IP_ALIGN);

bp->rx_offset - bp->rx_dma_offset == NET_IP_ALIGN, so data - NET_IP_ALIGN
is rx_buf->mapping. When NET_IP_ALIGN == 2, bnxt_init_one_rx_ring_rxbd()
sets RX_BD_FLAGS_SOP, and the hardware places the packet at mapping + 2.

Take a copybreak packet whose len is rx_copybreak or rx_copybreak - 1. The
memcpy reads up to rx_copybreak + 2 bytes from the mapping, so the last 1-2
bytes of the packet were never synced.

Could those bytes come from the stale original buffer with swiotlb, or from
a stale cache line with non-coherent DMA? bnxt reports
CHECKSUM_UNNECESSARY for these packets, so the stack would not catch the
corrupted tail.

This only affects architectures that keep NET_IP_ALIGN at 2. x86, arm64 and
powerpc define it as 0.

A sync length of len + NET_IP_ALIGN would fix this and would also bound the
overrun described above.

 		}
 		return 0;
 	case ETHTOOL_PFC_PREVENTION_TOUT:

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