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FreeBSD-EN-25:11.ena Errata Notice
The FreeBSD Project
Topic: ena resets and kernel panic on Nitro v4 or newer instances
Category: core
Module: ena
Announced: 2025-07-02
Credits: Arthur Kiyanovski
Affects: FreeBSD 13.5 and FreeBSD 14.2
Corrected: 2025-05-01 17:56:11 UTC (stable/14, 14.3-STABLE)
2025-07-02 18:28:12 UTC (releng/14.2, 14.2-RELEASE-p4)
2025-05-01 18:15:18 UTC (stable/13, 13.5-STABLE)
2025-07-02 18:28:31 UTC (releng/13.5, 13.5-RELEASE-p2)
For general information regarding FreeBSD Errata Notices and Security
Advisories, including descriptions of the fields above, security
branches, and the following sections, please visit
.
I. Background
The ena(4) driver is used to access the Elastic Network Adapter network
interface on recent Amazon Elastic Compute Cloud (EC2) instances. It is
designed to make full use of the EC2 cloud architecture for optimal network
performance.
ENA Express is a feature that allows increased bandwidth and reduced latency
in the AWS cloud. For optimal performance of the ENA Express feature, it is
necessary to reduce LLQ width to 128.
AWS instances that use Nitro card v4 or newer have a maximum tx burst size
when sending tx packets. The driver is responsible to adhere to this maximum
burst size by sending a doorbell to the device with no more than this burst
size packets. If the burst size is exceeded a device reset happens.
Since driver 2.8.0 it is possible to change the width of the tx queue LLQ
(Low Latency Queues) entries. There are 2 possible widths: 128 and 256 bytes.
The default is 256, however in some cases, i.e. when using the ENA Express
feature, it is recommended to use a width of 128.
II. Problem Description
When running on instances that have a max tx burst size and the ENA device
supports 256-byte wide LLQ entries, if 128-byte wide entries are selected,
either by setting hw.ena.force_large_llq_header = 0 via sysctl or by turning
on ENA Express for the interface, the ena(4) driver does not initialize a
stack variable which is later used to setup the maximum tx burst size.
III. Impact
Due to the uninitialized stack variable, the ena(4) driver will exceed the
maximum tx burst size, leading to device resets, making the device unusable.
Additionally, the calculation of the tx burst size includes division by the
uninitialized stack variable. If the stack variable is 0, this will cause
division by 0 in the kernel, leading to a kernel panic.
IV. Workaround
It is possible to force the LLQ width to 256 by setting
hw.ena.force_large_llq_header=1 via sysctl, however this causes peformance
degredation when using the ENA Express feature.
V. Solution
Upgrade your system to a supported FreeBSD stable or release / security
branch (releng) dated after the correction date and reboot.
Perform one of the following:
1) To update your system via a binary patch:
Systems running a RELEASE version of FreeBSD on the amd64 or arm64 platforms,
or the i386 platform on FreeBSD 13, can be updated via the freebsd-update(8)
utility:
# freebsd-update fetch
# freebsd-update install
# shutdown -r +10min "Rebooting for erratum update"
2) To update your system via a source code patch:
The following patches have been verified to apply to the applicable
FreeBSD release branches.
a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.
# fetch https://security.FreeBSD.org/patches/EN-25:11/ena.patch
# fetch https://security.FreeBSD.org/patches/EN-25:11/ena.patch.asc
# gpg --verify ena.patch.asc
b) Apply the patch. Execute the following commands as root:
# cd /usr/src
# patch < /path/to/patch
c) Recompile your kernel as described in
and reboot the
system.
VI. Correction details
This issue is corrected as of the corresponding Git commit hash in the
following stable and release branches:
Branch/path Hash Revision
- -------------------------------------------------------------------------
stable/14/ 3f4a674a8ee4 stable/14-n271320
releng/14.2/ ca1f7650a80d releng/14.2-n269528
stable/13/ 162b5bbb4048 stable/13-n259268
releng/13.5/ 575644144d5c releng/13.5-n259170
- -------------------------------------------------------------------------
Run the following command to see which files were modified by a
particular commit:
# git show --stat
Or visit the following URL, replacing NNNNNN with the hash:
To determine the commit count in a working tree (for comparison against
nNNNNNN in the table above), run:
# git rev-list --count --first-parent HEAD
VII. References
The latest revision of this advisory is available at
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