CVE-2026-99081: bnxt_en: fix DMA mapping length for padded small packets

CVE-2026-99081: bnxt_en: fix DMA mapping length for padded small packets

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

bnxt_en: fix DMA mapping length for padded small packets

Stefan Fleischmann reported Intel IOMMU DMA Read faults on BCM57412
NetXtreme-E NICs when transmitting packets on VLAN/macvlan interfaces:

  DMAR: [DMA Read NO_PASID] Request device [18:00.0] fault addr 0xfc499000
        [fault reason 0x06] PTE Read access is not set
  bnxt_en 0000:18:00.0 eno1np0: Abandoning msg {0xb4 0x41a} len: 0 due to firmware status: 0x2000001
  ...
  NETDEV WATCHDOG: eno1np0 (bnxt_en): transmit queue 0 timed out

The fault address (0xfc499000) is on an exact 4KB page boundary,
pointing to a DMA read buffer overrun.

In bnxt_start_xmit(), packets smaller than BNXT_MIN_PKT_SIZE (52 bytes),
such as 42-byte untagged ARP frames, are padded:

    if (length < BNXT_MIN_PKT_SIZE) {
        pad = BNXT_MIN_PKT_SIZE - length;
        if (skb_pad(skb, pad))
            goto tx_kick_pending;
        length = BNXT_MIN_PKT_SIZE;
    }

    mapping = dma_map_single(&pdev->dev, skb->data, len, DMA_TO_DEVICE);
    ...
    dma_unmap_len_set(tx_buf, len, len);

However, 'len' was initialized earlier to skb_headlen(skb) (e.g. 42 bytes)
and is left unadjusted after padding. Consequently, dma_map_single() and
dma_unmap_len_set() map and track only 42 bytes.

Later, the hardware TX buffer descriptor is programmed with the padded length:

    txbd->tx_bd_len_flags_type =
        cpu_to_le32(((len + pad) << TX_BD_LEN_SHIFT) | flags |
                    TX_BD_FLAGS_PACKET_END);

The NIC DMA engine is thus instructed to read 52 bytes from a region where
only 42 bytes were DMA-mapped. If skb->data ends near the boundary of a 4KB
page (within 'pad' bytes of the next page), the hardware DMA read overruns
into the unmapped adjacent page, triggering an IOMMU fault.

This issue was exposed after commit 447cbe95ebb9 ("vlan: fix skb_under_panic
and races when toggling HW VLAN offload") because reserving extra VLAN
headroom rounded LL_RESERVED_SPACE from 48 up to 64 bytes, shifting skb->data
offsets and potentially causing small frames to land right against page
boundaries.

Fix this by using skb_put_padto(skb, BNXT_MIN_PKT_SIZE) in
bnxt_start_xmit(), before skb_shinfo(skb)->nr_frags is sampled,
because padding might linearize the skb.
This ensures skb->len and skb_headlen(skb) reflect the padded size so
that dma_map_single() maps the full buffer and the descriptor length is
consistent. This also removes the temporary 'pad' variable and masking logic.

The padding is done after the SW USO branch, otherwise
bnxt_sw_udp_gso_xmit() would account the padding as UDP payload
and send it in extra segments.

Note that SW USO segments are still not padded: with IPv4, a segment
carrying less than 10 bytes of UDP payload (small gso_size, or a short
last segment) is sent as a frame shorter than BNXT_MIN_PKT_SIZE.
This is a separate issue, left for a followup patch.

The Linux kernel CVE team has assigned CVE-2026-99081 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 4.4 with commit c0c050c58d840994ba842ad1c338a98e7c12b764 and fixed in 6.6.160 with commit cf8530a25ae92c4493e2f22981ecb46f0b66f4c8
	Issue introduced in 4.4 with commit c0c050c58d840994ba842ad1c338a98e7c12b764 and fixed in 6.12.113 with commit 2956aca14dd2977cd08bd2ccafae3beb8b84b268
	Issue introduced in 4.4 with commit c0c050c58d840994ba842ad1c338a98e7c12b764 and fixed in 6.18.57 with commit fbd977e513971ccb4f0762a7f59eac500e474f89
	Issue introduced in 4.4 with commit c0c050c58d840994ba842ad1c338a98e7c12b764 and fixed in 7.2.11 with commit 5213f65a9667d3dd85db2e6df8f2e0516290db34
	Issue introduced in 4.4 with commit c0c050c58d840994ba842ad1c338a98e7c12b764 and fixed in 7.3 with commit a51d233ccd4889aadb71dace863525cc2b00271e

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-99081
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/net/ethernet/broadcom/bnxt/bnxt.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/cf8530a25ae92c4493e2f22981ecb46f0b66f4c8
	https://git.kernel.org/stable/c/2956aca14dd2977cd08bd2ccafae3beb8b84b268
	https://git.kernel.org/stable/c/fbd977e513971ccb4f0762a7f59eac500e474f89
	https://git.kernel.org/stable/c/5213f65a9667d3dd85db2e6df8f2e0516290db34
	https://git.kernel.org/stable/c/a51d233ccd4889aadb71dace863525cc2b00271e