net: sched: prevent UAF on tc_ctl_tfilter when temporarily dropping rtnl_lock

When dropping the rtnl_lock for looking up for a module, the device may be
removed, releasing the qdisc and class memory. Right after trying to load
the module, cl_ops->put is called, leading to a potential use-after-free.

Though commit e368fdb61d8e ("net: sched: use Qdisc rcu API instead of
relying on rtnl lock") fixes this, it involves a lot of refactoring of the
net/sched/ code, complicating its backport.

This fix calls cl_ops->put before dropping rtnl_lock as it will be called
either way, and zeroes it out so it won't be called again on the exit path.

This has been shown to stop the following KASAN report with the reproducer:

[  256.609111] ==================================================================
[  256.609585] BUG: KASAN: use-after-free in cbq_put+0x20/0xd0 at addr ffff880021daaba0
[  256.610078] Read of size 4 by task total_cbq/11184
[  256.610380] CPU: 0 PID: 11184 Comm: total_cbq Not tainted 4.9.311 #78
[  256.610778]  ffff8800215875a8 ffffffff96e18735 ffff880024803080 ffff880021daaa80
[  256.611274]  ffff8800215875d0 ffffffff96334841 ffffed00043b5574 ffffed00043b5574
[  256.611768]  ffff880024803080 ffff880021587658 ffffffff96334af8 0000000000000000
[  256.612186] Call Trace:
[  256.612344]  [<ffffffff96e18735>] dump_stack+0x6d/0x8b
[  256.612632]  [<ffffffff96334841>] kasan_object_err+0x21/0x70
[  256.612973]  [<ffffffff96334af8>] kasan_report.part.1+0x218/0x4f0
[  256.613349]  [<ffffffff96c5a2e0>] ? cbq_put+0x20/0xd0
[  256.613634]  [<ffffffff96333cd6>] ? kasan_unpoison_shadow+0x36/0x50
[  256.613993]  [<ffffffff96335105>] kasan_report+0x25/0x30
[  256.614288]  [<ffffffff96333701>] __asan_load4+0x61/0x80
[  256.614580]  [<ffffffff96c5a2e0>] cbq_put+0x20/0xd0
[  256.614862]  [<ffffffff96c53184>] tc_ctl_tfilter+0x4f4/0xb80
[  256.615151]  [<ffffffff96c52c90>] ? tfilter_notify+0x140/0x140
[  256.615478]  [<ffffffff960056ef>] ? do_syscall_64+0xef/0x190
[  256.615799]  [<ffffffff96e28a8e>] ? entry_SYSCALL_64_after_swapgs+0x58/0xc6
[  256.616190]  [<ffffffff96bce3f6>] ? sock_sendmsg+0x76/0x80
[  256.616484]  [<ffffffff96bce53f>] ? sock_write_iter+0x13f/0x1f0
[  256.616833]  [<ffffffff96367b02>] ? __vfs_write+0x262/0x3c0
[  256.617152]  [<ffffffff96369dc9>] ? vfs_write+0xf9/0x260
[  256.617451]  [<ffffffff9636c009>] ? SyS_write+0xc9/0x1b0
[  256.617754]  [<ffffffff960decda>] ? ns_capable_common+0x5a/0xa0
[  256.618067]  [<ffffffff960ded33>] ? ns_capable+0x13/0x20
[  256.618334]  [<ffffffff96c9125d>] ? __netlink_ns_capable+0x6d/0x80
[  256.618666]  [<ffffffff96c2750f>] rtnetlink_rcv_msg+0x1af/0x410
[  256.618969]  [<ffffffff96c90d6b>] ? netlink_compare+0x5b/0x70
[  256.619295]  [<ffffffff96c27360>] ? rtnl_newlink+0xc60/0xc60
[  256.619587]  [<ffffffff96c94214>] ? __netlink_lookup+0x1a4/0x240
[  256.619885]  [<ffffffff96c94070>] ? netlink_broadcast+0x20/0x20
[  256.620179]  [<ffffffff96c97815>] netlink_rcv_skb+0x155/0x190
[  256.620463]  [<ffffffff96c27360>] ? rtnl_newlink+0xc60/0xc60
[  256.620748]  [<ffffffff96c1e758>] rtnetlink_rcv+0x28/0x30
[  256.621015]  [<ffffffff96c96d11>] netlink_unicast+0x2f1/0x3b0
[  256.621354]  [<ffffffff96c96a20>] ? netlink_attachskb+0x340/0x340
[  256.621765]  [<ffffffff96c9733e>] netlink_sendmsg+0x56e/0x6f0
[  256.622181]  [<ffffffff96c96dd0>] ? netlink_unicast+0x3b0/0x3b0
[  256.622578]  [<ffffffff96c96dd0>] ? netlink_unicast+0x3b0/0x3b0
[  256.622893]  [<ffffffff96bce3f6>] sock_sendmsg+0x76/0x80
[  256.623157]  [<ffffffff96bce53f>] sock_write_iter+0x13f/0x1f0
[  256.623440]  [<ffffffff96bce400>] ? sock_sendmsg+0x80/0x80
[  256.623729]  [<ffffffff966a8032>] ? iov_iter_init+0x82/0xc0
[  256.624006]  [<ffffffff96367b02>] __vfs_write+0x262/0x3c0
[  256.624274]  [<ffffffff963678a0>] ? default_llseek+0x120/0x120
[  256.624566]  [<ffffffff965e8c02>] ? common_file_perm+0x92/0x170
[  256.624925]  [<ffffffff96369a58>] ? rw_verify_area+0x78/0x140
[  256.625277]  [<ffffffff96369dc9>] vfs_write+0xf9/0x260
[  256.625593]  [<ffffffff9636c009>] SyS_write+0xc9/0x1b0
[  256.625891]  [<ffffffff9636bf40>] ? SyS_read+0x1b0/0x1b0
[  256.626154]  [<ffffffff9636bf40>] ? SyS_read+0x1b0/0x1b0
[  256.626422]  [<ffffffff960056ef>] do_syscall_64+0xef/0x190
[  256.626697]  [<ffffffff96e28a8e>] entry_SYSCALL_64_after_swapgs+0x58/0xc6
[  256.627033] Object at ffff880021daaa80, in cache kmalloc-512 size: 512
[  256.627415] Allocated:
[  256.627563] PID = 164
[  256.627711]  save_stack_trace+0x1b/0x20
[  256.627947]  save_stack+0x46/0xd0
[  256.628151]  kasan_kmalloc+0xad/0xe0
[  256.628362]  kmem_cache_alloc_trace+0xe8/0x1e0
[  256.628637]  cbq_change_class+0x8b6/0xde0
[  256.628896]  tc_ctl_tclass+0x56a/0x5b0
[  256.629129]  rtnetlink_rcv_msg+0x1af/0x410
[  256.629380]  netlink_rcv_skb+0x155/0x190
[  256.629621]  rtnetlink_rcv+0x28/0x30
[  256.629840]  netlink_unicast+0x2f1/0x3b0
[  256.630066]  netlink_sendmsg+0x56e/0x6f0
[  256.630263]  sock_sendmsg+0x76/0x80
[  256.630456]  sock_write_iter+0x13f/0x1f0
[  256.630698]  __vfs_write+0x262/0x3c0
[  256.630918]  vfs_write+0xf9/0x260
[  256.631123]  SyS_write+0xc9/0x1b0
[  256.631327]  do_syscall_64+0xef/0x190
[  256.631553]  entry_SYSCALL_64_after_swapgs+0x58/0xc6
[  256.631827] Freed:
[  256.631931] PID = 164
[  256.632048]  save_stack_trace+0x1b/0x20
[  256.632241]  save_stack+0x46/0xd0
[  256.632408]  kasan_slab_free+0x71/0xb0
[  256.632597]  kfree+0x8c/0x1a0
[  256.632751]  cbq_destroy_class+0x85/0xa0
[  256.632948]  cbq_destroy+0xfa/0x120
[  256.633125]  qdisc_destroy+0xa1/0x140
[  256.633309]  dev_shutdown+0x12d/0x190
[  256.633497]  rollback_registered_many+0x43c/0x5b0
[  256.633753]  unregister_netdevice_many+0x2c/0x130
[  256.634041]  rtnl_delete_link+0xb3/0x100
[  256.634283]  rtnl_dellink+0x19c/0x360
[  256.634509]  rtnetlink_rcv_msg+0x1af/0x410
[  256.634760]  netlink_rcv_skb+0x155/0x190
[  256.635001]  rtnetlink_rcv+0x28/0x30
[  256.635221]  netlink_unicast+0x2f1/0x3b0
[  256.635463]  netlink_sendmsg+0x56e/0x6f0
[  256.635700]  sock_sendmsg+0x76/0x80
[  256.635915]  sock_write_iter+0x13f/0x1f0
[  256.636156]  __vfs_write+0x262/0x3c0
[  256.636376]  vfs_write+0xf9/0x260
[  256.636580]  SyS_write+0xc9/0x1b0
[  256.636787]  do_syscall_64+0xef/0x190
[  256.637013]  entry_SYSCALL_64_after_swapgs+0x58/0xc6
[  256.637316] Memory state around the buggy address:
[  256.637610]  ffff880021daaa80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[  256.638047]  ffff880021daab00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[  256.638487] >ffff880021daab80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[  256.638924]                                ^
[  256.639186]  ffff880021daac00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[  256.639624]  ffff880021daac80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>