sch_cbq: validate TCA_CBQ_WRROPT to avoid crash

syzbot reported a crash in cbq_normalize_quanta() caused
by an out of range cl->priority.

iproute2 enforces this check, but malicious users do not.

kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] SMP KASAN PTI
Modules linked in:
CPU: 1 PID: 26447 Comm: syz-executor.1 Not tainted 5.3+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:cbq_normalize_quanta.part.0+0x1fd/0x430 net/sched/sch_cbq.c:902
RSP: 0018:ffff8801a5c333b0 EFLAGS: 00010206
RAX: 0000000020000003 RBX: 00000000fffffff8 RCX: ffffc9000712f000
RDX: 00000000000043bf RSI: ffffffff83be8962 RDI: 0000000100000018
RBP: ffff8801a5c33420 R08: 000000000000003a R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 00000000000002ef
R13: ffff88018da95188 R14: dffffc0000000000 R15: 0000000000000015
FS:  00007f37d26b1700(0000) GS:ffff8801dad00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000004c7cec CR3: 00000001bcd0a006 CR4: 00000000001626f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 [<ffffffff83be9d57>] cbq_normalize_quanta include/net/pkt_sched.h:27 [inline]
 [<ffffffff83be9d57>] cbq_addprio net/sched/sch_cbq.c:1097 [inline]
 [<ffffffff83be9d57>] cbq_set_wrr+0x2d7/0x450 net/sched/sch_cbq.c:1115
 [<ffffffff83bee8a7>] cbq_change_class+0x987/0x225b net/sched/sch_cbq.c:1537
 [<ffffffff83b96985>] tc_ctl_tclass+0x555/0xcd0 net/sched/sch_api.c:2329
 [<ffffffff83a84655>] rtnetlink_rcv_msg+0x485/0xc10 net/core/rtnetlink.c:5248
 [<ffffffff83cadf0a>] netlink_rcv_skb+0x17a/0x460 net/netlink/af_netlink.c:2510
 [<ffffffff83a7db6d>] rtnetlink_rcv+0x1d/0x30 net/core/rtnetlink.c:5266
 [<ffffffff83cac2c6>] netlink_unicast_kernel net/netlink/af_netlink.c:1324 [inline]
 [<ffffffff83cac2c6>] netlink_unicast+0x536/0x720 net/netlink/af_netlink.c:1350
 [<ffffffff83cacd4a>] netlink_sendmsg+0x89a/0xd50 net/netlink/af_netlink.c:1939
 [<ffffffff8399d46e>] sock_sendmsg_nosec net/socket.c:673 [inline]
 [<ffffffff8399d46e>] sock_sendmsg+0x12e/0x170 net/socket.c:684
 [<ffffffff8399f1fd>] ___sys_sendmsg+0x81d/0x960 net/socket.c:2359
 [<ffffffff839a2d05>] __sys_sendmsg+0x105/0x1d0 net/socket.c:2397
 [<ffffffff839a2df9>] SYSC_sendmsg net/socket.c:2406 [inline]
 [<ffffffff839a2df9>] SyS_sendmsg+0x29/0x30 net/socket.c:2404
 [<ffffffff8101ccc8>] do_syscall_64+0x528/0x770 arch/x86/entry/common.c:305
 [<ffffffff84400091>] entry_SYSCALL_64_after_hwframe+0x42/0xb7

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_cbq.c

@@ -1127,6 +1127,33 @@ static const struct nla_policy cbq_policy[TCA_CBQ_MAX + 1] = {
 	[TCA_CBQ_POLICE]	= { .len = sizeof(struct tc_cbq_police) },
 };
 
+static int cbq_opt_parse(struct nlattr *tb[TCA_CBQ_MAX + 1],
+			 struct nlattr *opt,
+			 struct netlink_ext_ack *extack)
+{
+	int err;
+
+	if (!opt) {
+		NL_SET_ERR_MSG(extack, "CBQ options are required for this operation");
+		return -EINVAL;
+	}
+
+	err = nla_parse_nested_deprecated(tb, TCA_CBQ_MAX, opt,
+					  cbq_policy, extack);
+	if (err < 0)
+		return err;
+
+	if (tb[TCA_CBQ_WRROPT]) {
+		const struct tc_cbq_wrropt *wrr = nla_data(tb[TCA_CBQ_WRROPT]);
+
+		if (wrr->priority > TC_CBQ_MAXPRIO) {
+			NL_SET_ERR_MSG(extack, "priority is bigger than TC_CBQ_MAXPRIO");
+			err = -EINVAL;
+		}
+	}
+	return err;
+}
+
 static int cbq_init(struct Qdisc *sch, struct nlattr *opt,
 		    struct netlink_ext_ack *extack)
 {
@@ -1139,13 +1166,7 @@ static int cbq_init(struct Qdisc *sch, struct nlattr *opt,
 	hrtimer_init(&q->delay_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_PINNED);
 	q->delay_timer.function = cbq_undelay;
 
-	if (!opt) {
-		NL_SET_ERR_MSG(extack, "CBQ options are required for this operation");
-		return -EINVAL;
-	}
-
-	err = nla_parse_nested_deprecated(tb, TCA_CBQ_MAX, opt, cbq_policy,
-					  extack);
+	err = cbq_opt_parse(tb, opt, extack);
 	if (err < 0)
 		return err;
 
@@ -1464,13 +1485,7 @@ cbq_change_class(struct Qdisc *sch, u32 classid, u32 parentid, struct nlattr **t
 	struct cbq_class *parent;
 	struct qdisc_rate_table *rtab = NULL;
 
-	if (!opt) {
-		NL_SET_ERR_MSG(extack, "Mandatory qdisc options missing");
-		return -EINVAL;
-	}
-
-	err = nla_parse_nested_deprecated(tb, TCA_CBQ_MAX, opt, cbq_policy,
-					  extack);
+	err = cbq_opt_parse(tb, opt, extack);
 	if (err < 0)
 		return err;