[PATCH net] net: dsa: tag_brcm: legacy FCS: request needed tailroom

[PATCH net] net: dsa: tag_brcm: legacy FCS: request needed tailroom

[Weiming Shi]

The legacy FCS tagger calculates the CRC over skb->len bytes starting at
skb->data. When a nonlinear skb reaches the tagger, this reads past the
linear head into unrelated slab memory.

The tagger appends an Ethernet FCS but does not declare that tailroom. As a
result, DSA leaves NETIF_F_SG and NETIF_F_FRAGLIST enabled on the user
port, and nonlinear skbs can reach the CRC calculation.

Declare the required tailroom. DSA will then clear those features and the
networking core will linearize skbs before the tagger runs.

A KASAN-enabled dsa_loop test using this tagger reports:

  BUG: KASAN: slab-out-of-bounds in crc32_le
  Read of size 1 at addr ffff8880397086c0 by task exp/135

  Call Trace:
   crc32_le (lib/crc/crc32-main.c:38)
   brcm_leg_fcs_tag_xmit (net/dsa/tag_brcm.c:343)
   dsa_user_xmit (net/dsa/user.c:942)
   dev_hard_start_xmit (net/core/dev.c:3937)
   __dev_queue_xmit (net/core/dev.c:4926)
   packet_sendmsg (net/packet/af_packet.c:3110)
   __sys_sendto (net/socket.c:2281)

  The buggy address belongs to the object at ffff888039708400
   which belongs to the cache skbuff_small_head of size 704
  The buggy address is located 0 bytes to the right of
   allocated 704-byte region [ffff888039708400, ffff8880397086c0)

Fixes: ef07df397a62 ("net: dsa: tag_brcm: add support for legacy FCS tags")
Cc: stable@vger.kernel.org
Reported-by: co+28eef7d8af9428e6@bugs.sh
Closes: https://lore.kernel.org/all/jH6u350kaBRuqklDjd3k3BW4nWzp0tYRjq3p%40bugs.sh/
Assisted-by: Claude:gpt-5
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
 net/dsa/tag_brcm.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/dsa/tag_brcm.c b/net/dsa/tag_brcm.c
index 411e3b57d16af..b7c49822ca888 100644
--- a/net/dsa/tag_brcm.c
+++ b/net/dsa/tag_brcm.c
@@ -373,6 +373,7 @@ static const struct dsa_device_ops brcm_legacy_fcs_netdev_ops = {
 	.xmit = brcm_leg_fcs_tag_xmit,
 	.rcv = brcm_leg_tag_rcv,
 	.needed_headroom = BRCM_LEG_TAG_LEN,
+	.needed_tailroom = ETH_FCS_LEN,
 };
 
 DSA_TAG_DRIVER(brcm_legacy_fcs_netdev_ops);
-- 
2.55.0

Re: [PATCH net] net: dsa: tag_brcm: legacy FCS: request needed tailroom

[Florian Fainelli]

On 9/8/26 09:50, Weiming Shi wrote:
> The legacy FCS tagger calculates the CRC over skb->len bytes starting at
> skb->data. When a nonlinear skb reaches the tagger, this reads past the
> linear head into unrelated slab memory.
> 
> The tagger appends an Ethernet FCS but does not declare that tailroom. As a
> result, DSA leaves NETIF_F_SG and NETIF_F_FRAGLIST enabled on the user
> port, and nonlinear skbs can reach the CRC calculation.
> 
> Declare the required tailroom. DSA will then clear those features and the
> networking core will linearize skbs before the tagger runs.
> 
> A KASAN-enabled dsa_loop test using this tagger reports:
> 
>    BUG: KASAN: slab-out-of-bounds in crc32_le
>    Read of size 1 at addr ffff8880397086c0 by task exp/135
> 
>    Call Trace:
>     crc32_le (lib/crc/crc32-main.c:38)
>     brcm_leg_fcs_tag_xmit (net/dsa/tag_brcm.c:343)
>     dsa_user_xmit (net/dsa/user.c:942)
>     dev_hard_start_xmit (net/core/dev.c:3937)
>     __dev_queue_xmit (net/core/dev.c:4926)
>     packet_sendmsg (net/packet/af_packet.c:3110)
>     __sys_sendto (net/socket.c:2281)
> 
>    The buggy address belongs to the object at ffff888039708400
>     which belongs to the cache skbuff_small_head of size 704
>    The buggy address is located 0 bytes to the right of
>     allocated 704-byte region [ffff888039708400, ffff8880397086c0)
> 
> Fixes: ef07df397a62 ("net: dsa: tag_brcm: add support for legacy FCS tags")
> Cc: stable@vger.kernel.org
> Reported-by: co+28eef7d8af9428e6@bugs.sh
> Closes: https://lore.kernel.org/all/jH6u350kaBRuqklDjd3k3BW4nWzp0tYRjq3p%40bugs.sh/
> Assisted-by: Claude:gpt-5
> Signed-off-by: Weiming Shi <bestswngs@gmail.com>

Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
-- 
Florian

Re: [PATCH net] net: dsa: tag_brcm: legacy FCS: request needed tailroom

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Wed,  9 Sep 2026 00:50:47 +0800 you wrote:
> The legacy FCS tagger calculates the CRC over skb->len bytes starting at
> skb->data. When a nonlinear skb reaches the tagger, this reads past the
> linear head into unrelated slab memory.
> 
> The tagger appends an Ethernet FCS but does not declare that tailroom. As a
> result, DSA leaves NETIF_F_SG and NETIF_F_FRAGLIST enabled on the user
> port, and nonlinear skbs can reach the CRC calculation.
> 
> [...]

Here is the summary with links:
  - [net] net: dsa: tag_brcm: legacy FCS: request needed tailroom
    https://git.kernel.org/netdev/net/c/5be081b83abd

You are awesome, thank you!
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