netdev - Re: [PATCH v2][next] virtio_net: Fix misalignment bug in struct virtnet

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <e9607915-892c-4724-b97f-7c90918f86fe@redhat.com>
Date: Tue, 13 Jan 2026 15:30:00 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>,
 "Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>,
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez
 <eperezma@...hat.com>, Andrew Lunn <andrew+netdev@...n.ch>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Akihiko Odaki <akihiko.odaki@...nix.com>,
 Simon Horman <horms@...nel.org>
Cc: netdev@...r.kernel.org, virtualization@...ts.linux.dev,
 linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
 Kees Cook <kees@...nel.org>
Subject: Re: [PATCH v2][next] virtio_net: Fix misalignment bug in struct
 virtnet_info

On 1/10/26 9:07 AM, Gustavo A. R. Silva wrote:
> Use the new TRAILING_OVERLAP() helper to fix a misalignment bug
> along with the following warning:
> 
> drivers/net/virtio_net.c:429:46: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 
> This helper creates a union between a flexible-array member (FAM)
> and a set of members that would otherwise follow it (in this case
> `u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];`). This
> overlays the trailing members (rss_hash_key_data) onto the FAM
> (hash_key_data) while keeping the FAM and the start of MEMBERS aligned.
> The static_assert() ensures this alignment remains.
> 
> Notice that due to tail padding in flexible `struct
> virtio_net_rss_config_trailer`, `rss_trailer.hash_key_data`
> (at offset 83 in struct virtnet_info) and `rss_hash_key_data` (at
> offset 84 in struct virtnet_info) are misaligned by one byte. See
> below:
> 
> struct virtio_net_rss_config_trailer {
>         __le16                     max_tx_vq;            /*     0     2 */
>         __u8                       hash_key_length;      /*     2     1 */
>         __u8                       hash_key_data[];      /*     3     0 */
> 
>         /* size: 4, cachelines: 1, members: 3 */
>         /* padding: 1 */
>         /* last cacheline: 4 bytes */
> };
> 
> struct virtnet_info {
> ...
>         struct virtio_net_rss_config_trailer rss_trailer; /*    80     4 */
> 
>         /* XXX last struct has 1 byte of padding */
> 
>         u8                         rss_hash_key_data[40]; /*    84    40 */
> ...
>         /* size: 832, cachelines: 13, members: 48 */
>         /* sum members: 801, holes: 8, sum holes: 31 */
>         /* paddings: 2, sum paddings: 5 */
> };
> 
> After changes, those members are correctly aligned at offset 795:
> 
> struct virtnet_info {
> ...
>         union {
>                 struct virtio_net_rss_config_trailer rss_trailer; /*   792     4 */
>                 struct {
>                         unsigned char __offset_to_hash_key_data[3]; /*   792     3 */
>                         u8         rss_hash_key_data[40]; /*   795    40 */
>                 };                                       /*   792    43 */
>         };                                               /*   792    44 */
> ...
>         /* size: 840, cachelines: 14, members: 47 */
>         /* sum members: 801, holes: 8, sum holes: 35 */
>         /* padding: 4 */
>         /* paddings: 1, sum paddings: 4 */
>         /* last cacheline: 8 bytes */
> };
> 
> As a result, the RSS key passed to the device is shifted by 1
> byte: the last byte is cut off, and instead a (possibly
> uninitialized) byte is added at the beginning.
> 
> As a last note `struct virtio_net_rss_config_hdr *rss_hdr;` is also
> moved to the end, since it seems those three members should stick
> around together. :)
> 
> Cc: stable@...r.kernel.org
> Fixes: ed3100e90d0d ("virtio_net: Use new RSS config structs")
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> ---
> Changes in v2:
>  - Update subject and changelog text (include feedback from Simon and
>    Michael --thanks folks)
>  - Add Fixes tag and CC -stable.

@Michael, @Jason: This is still apparently targeting 'net-next', but I
think it should land in the 'net' tree, right?

/P