[PATCH 6.18 096/277] net/sched: cls_fw: fix NULL pointer dereference on shared blocks

Greg Kroah-Hartman
2026-04-08T18:31:28Z

From: Xiang Mei <xmei5@asu.edu>

[ Upstream commit faeea8bbf6e958bf3c00cb08263109661975987c ]

The old-method path in fw_classify() calls tcf_block_q() and
dereferences q->handle. Shared blocks leave block->q NULL, causing a
NULL deref when an empty cls_fw filter is attached to a shared block
and a packet with a nonzero major skb mark is classified.

Reject the configuration in fw_change() when the old method (no
TCA_OPTIONS) is used on a shared block, since fw_classify()'s
old-method path needs block->q which is NULL for shared blocks.

The fixed null-ptr-deref calling stack:
KASAN: null-ptr-deref in range [0x0000000000000038-0x000000000000003f]
RIP: 0010:fw_classify (net/sched/cls_fw.c:81)
Call Trace:
tcf_classify (./include/net/tc_wrapper.h:197 net/sched/cls_api.c:1764 net/sched/cls_api.c:1860)
tc_run (net/core/dev.c:4401)
__dev_queue_xmit (net/core/dev.c:4535 net/core/dev.c:4790)

Fixes: 1abf272022cf ("net: sched: tcindex, fw, flow: use tcf_block_q helper to get struct Qdisc")
Reported-by: Weiming Shi <bestswngs@gmail.com>
Signed-off-by: Xiang Mei <xmei5@asu.edu>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20260331050217.504278-1-xmei5@asu.edu
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/sched/cls_fw.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/net/sched/cls_fw.c b/net/sched/cls_fw.c
index cdddc86952284..83a7372ea15c2 100644
--- a/net/sched/cls_fw.c
+++ b/net/sched/cls_fw.c
@@ -247,8 +247,18 @@ static int fw_change(struct net *net, struct sk_buff *in_skb,
struct nlattr *tb[TCA_FW_MAX + 1];
int err;

- if (!opt)
- return handle ? -EINVAL : 0; /* Succeed if it is old method. */
+ if (!opt) {
+ if (handle)
+ return -EINVAL;
+
+ if (tcf_block_shared(tp->chain->block)) {
+ NL_SET_ERR_MSG(extack,
+ "Must specify mark when attaching fw filter to block");
+ return -EINVAL;
+ }
+
+ return 0; /* Succeed if it is old method. */
+ }

err = nla_parse_nested_deprecated(tb, TCA_FW_MAX, opt, fw_policy,
NULL);
--
2.53.0