net: dsa: tag_brcm: legacy FCS: request needed tailroom

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: ef07df397a ("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/
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://patch.msgid.link/20260908165047.2786340-1-bestswngs@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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);