CVE-2026-74269: bnxt: fix head underflow on XDP head-grow

CVE-2026-74269: bnxt: fix head underflow on XDP head-grow

[Greg Kroah-Hartman]

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

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

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

bnxt: fix head underflow on XDP head-grow

The xdp.py test test_xdp_native_adjst_head_grow_data crashes when run on
a bnxt machine (and also crashes in NIPA).

It seems that the bug is an underflow in bnxt_rx_multi_page_skb, which
builds the skb head:

  napi_build_skb(data_ptr - bp->rx_offset, rxr->rx_page_size);

The problem with this expression is that in page mode, rx_offset is:

  bp->rx_offset = NET_IP_ALIGN + XDP_PACKET_HEADROOM;

Which evaluates (at least on x86_64) to 258.

The test test_xdp_native_adjst_head_grow_data tests a case where the
head is adjusted by -256.

When this test runs, data_ptr is shifted to frag_start + 2 (where
frag_start = page_address(page) + offset).

Then, bnxt_rx_multi_page_skb is invoked and the napi_build_skb
expression subtracts 258, landing at an address before frag_start. This
could be either the previous fragment or the previous physical page when
the offset is < 256 (e.g. if the fragment started at offset 0).

When the skb is freed, the page pool fragment reference is dropped on
either the wrong page or the wrong frag of the right page. In either
case, the corrupted reference count can lead to the page being
prematurely recycled while still in use. Once (incorrectly) recycled, it
can be handed out again and on driver teardown this would result in a
double free.

The commit under fixes updated this code to handle the case where the
native page size is >= 64k, but it unintentionally broke the head grow
case.

To fix this, add an offset field to struct bnxt_sw_rx_bd, mirroring the
existing offset field in struct bnxt_sw_rx_agg_bd. Populate it on
allocation and preserve it on reuse.

In bnxt_rx_multi_page_skb, use the newly added offset field to compute
the fragment start and pass that to napi_build_skb. Adjust the layout
with skb_reserve.

There are two cases, the non-adjustment case and the adjustment case.

In both cases, the skb is built at page_address(page) + offset to
account for the case where the native page size >= 64K and skb_reserve
is called with data_ptr - (page_address(page) + offset). That
difference equals bp->rx_offset when data_ptr was not moved, or
bp->rx_offset + xdp_adjust when XDP adjusted the head.

Re-running the failing test with this commit applied causes the test to
run successfully to completion.

The other rx_skb_func implementations don't have this issue.

The Linux kernel CVE team has assigned CVE-2026-74269 to this issue.


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

	Issue introduced in 6.5 with commit f6974b4c2d8e1062b5a52228ee47293c15b4ee1e and fixed in 7.1.5 with commit bb72b1c6755631c74b7e0878ee55bb81c06776c0
	Issue introduced in 6.5 with commit f6974b4c2d8e1062b5a52228ee47293c15b4ee1e and fixed in 7.2-rc1 with commit e26657fe3b85c068b01f42bb0c602f242d643ba9
	Issue introduced in 6.1.45 with commit e9f11bfc03fb0d3c86f91b8ae945bb10f7e19c16
	Issue introduced in 6.4.10 with commit ae0e135dc900827687ecc684c2bbb57aae48d318

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-2026-74269
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
	drivers/net/ethernet/broadcom/bnxt/bnxt.h


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/bb72b1c6755631c74b7e0878ee55bb81c06776c0
	https://git.kernel.org/stable/c/e26657fe3b85c068b01f42bb0c602f242d643ba9