---
id: "CVE-2026-99081"
kind: "vulnerability"
aliases: ["CVE-2026-99081"]
title: "Linux bnxt_en driver: short-packet padding maps too few bytes, causing DMA faults"
severity: "low"
cvss: null
epss: 0.00209
exploited: false
status: "open"
category: "infrastructure"
products: []
ecosystems: []
tags: ["bnxt_en", "broadcom", "infrastructure", "iommu", "kernel", "linux", "macvlan", "nic", "vlan"]
not_affected_if: ["no-bnxt-en-hardware", "non-linux"]
published_at: "2026-10-23T13:17:32+00:00"
first_seen_at: "2026-10-23T21:57:34+00:00"
updated_at: "2026-10-25T12:38:50+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-99081", "first_seen_at": "2026-10-23T21:57:34+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2026-99081", "first_seen_at": "2026-10-25T12:38:50+00:00"}]
---

# Linux bnxt_en driver: short-packet padding maps too few bytes, causing DMA faults

In the Linux kernel's bnxt_en driver for Broadcom NetXtreme-E NICs, packets shorter than 52 bytes are padded but the DMA mapping covers only the original length, so the NIC reads past the mapped buffer. The result is IOMMU DMA faults, adapter errors and transmit queue timeouts, a denial of service rather than code execution.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Run `uname -r` and check whether the kernel predates the fix commit, which the sources do not name.
- Run `lspci -nn | grep -i broadcom` to see whether a BCM57412 NetXtreme-E NIC is present.
- Look in `dmesg` for `DMAR: [DMA Read NO_PASID]` faults or `bnxt_en` messages such as `Abandoning msg` and `transmit queue 0 timed out`.

## Fix

- Update the Linux kernel to a release that includes the bnxt_en fix (the sources do not list the exact version: unknown).
- Until patched, there is no workaround stated in the sources: unknown.

## Timeline

- 2026-10-23T21:57:34+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-99081
- 2026-10-25T12:38:50+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2026-99081

## From the sources

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.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_