[PATCH net] net: sched: disallow noqueue for qdisc classes

Frederick Lawler
2023-01-09T16:39:41Z

While experimenting with applying noqueue to a classful queue discipline,
we discovered a NULL pointer dereference in the __dev_queue_xmit()
path that generates a kernel OOPS:

# dev=enp0s5
# tc qdisc replace dev $dev root handle 1: htb default 1
# tc class add dev $dev parent 1: classid 1:1 htb rate 10mbit
# tc qdisc add dev $dev parent 1:1 handle 10: noqueue
# ping -I $dev -w 1 -c 1 1.1.1.1

[ 2.172856] BUG: kernel NULL pointer dereference, address: 0000000000000000
[ 2.173217] #PF: supervisor instruction fetch in kernel mode
...
[ 2.178451] Call Trace:
[ 2.178577] <TASK>
[ 2.178686] htb_enqueue+0x1c8/0x370
[ 2.178880] dev_qdisc_enqueue+0x15/0x90
[ 2.179093] __dev_queue_xmit+0x798/0xd00
[ 2.179305] ? _raw_write_lock_bh+0xe/0x30
[ 2.179522] ? __local_bh_enable_ip+0x32/0x70
[ 2.179759] ? ___neigh_create+0x610/0x840
[ 2.179968] ? eth_header+0x21/0xc0
[ 2.180144] ip_finish_output2+0x15e/0x4f0
[ 2.180348] ? dst_output+0x30/0x30
[ 2.180525] ip_push_pending_frames+0x9d/0xb0
[ 2.180739] raw_sendmsg+0x601/0xcb0
[ 2.180916] ? _raw_spin_trylock+0xe/0x50
[ 2.181112] ? _raw_spin_unlock_irqrestore+0x16/0x30
[ 2.181354] ? get_page_from_freelist+0xcd6/0xdf0
[ 2.181594] ? sock_sendmsg+0x56/0x60
[ 2.181781] sock_sendmsg+0x56/0x60
[ 2.181958] __sys_sendto+0xf7/0x160
[ 2.182139] ? handle_mm_fault+0x6e/0x1d0
[ 2.182366] ? do_user_addr_fault+0x1e1/0x660
[ 2.182627] __x64_sys_sendto+0x1b/0x30
[ 2.182881] do_syscall_64+0x38/0x90
[ 2.183085] entry_SYSCALL_64_after_hwframe+0x63/0xcd
...
[ 2.187402] </TASK>

Previously in commit d66d6c3152e8 ("net: sched: register noqueue
qdisc"), NULL was set for the noqueue discipline on noqueue init
so that __dev_queue_xmit() falls through for the noqueue case. This
also sets a bypass of the enqueue NULL check in the
register_qdisc() function for the struct noqueue_disc_ops.

Classful queue disciplines make it past the NULL check in
__dev_queue_xmit() because the discipline is set to htb (in this case),
and then in the call to __dev_xmit_skb(), it calls into htb_enqueue()
which grabs a leaf node for a class and then calls qdisc_enqueue() by
passing in a queue discipline which assumes ->enqueue() is not set to NULL.

Fix this by not allowing classes to be assigned to the noqueue
discipline. Linux TC Notes states that classes cannot be set to
the noqueue discipline. [1] Let's enforce that here.

Links:
1. https://linux-tc-notes.sourceforge.net/tc/doc/sch_noqueue.txt

Fixes: d66d6c3152e8 ("net: sched: register noqueue qdisc")
Cc: stable@vger.kernel.org
Signed-off-by: Frederick Lawler <fred@cloudflare.com>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
---
net/sched/sch_api.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
index 2317db02c764..72d2c204d5f3 100644
--- a/net/sched/sch_api.c
+++ b/net/sched/sch_api.c
@@ -1133,6 +1133,11 @@ static int qdisc_graft(struct net_device *dev, struct Qdisc *parent,
return -ENOENT;
}

+ if (new && new->ops == &noqueue_qdisc_ops) {
+ NL_SET_ERR_MSG(extack, "Cannot assign noqueue to a class");
+ return -EINVAL;
+ }
+
err = cops->graft(parent, cl, new, &old, extack);
if (err)
return err;
--
2.34.1