From 5be081b83abd3f17d908953b4bb77279f5a149e3 Mon Sep 17 00:00:00 2001 From: Weiming Shi Date: Wed, 9 Sep 2026 00:50:47 +0800 Subject: [PATCH] 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: 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/ Signed-off-by: Weiming Shi Reviewed-by: Florian Fainelli Link: https://patch.msgid.link/20260908165047.2786340-1-bestswngs@gmail.com Signed-off-by: Jakub Kicinski --- 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 411e3b57d16a..b7c49822ca88 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);