bpf: net/core/filter.c:2115 suspicious rcu_dereference_protected() usage!

[Sasha Levin <sasha.levin@oracle.com>]

Hi all,

I've hit the following warning while fuzzing with trinity inside a kvmtool guest
running the latest -next kernel:

[ 1343.104588] ===============================

[ 1343.104591] [ INFO: suspicious RCU usage. ]

[ 1343.104619] 4.5.0-rc4-next-20160219-sasha-00026-g7978205-dirty #2978 Not tainted

[ 1343.104624] -------------------------------

[ 1343.104635] net/core/filter.c:2115 suspicious rcu_dereference_protected() usage!

[ 1343.104641]

[ 1343.104641] other info that might help us debug this:

[ 1343.104641]

[ 1343.104650]

[ 1343.104650] rcu_scheduler_active = 1, debug_locks = 0

[ 1343.104660] 1 lock held by syz-executor/17916:

[ 1343.104784] #0: (rtnl_mutex){+.+.+.}, at: rtnl_lock (net/core/rtnetlink.c:71)
[ 1343.104789]

[ 1343.104789] stack backtrace:

[ 1343.104820] CPU: 1 PID: 17916 Comm: trinity-c8 Not tainted 4.5.0-rc4-next-20160219-sasha-00026-g7978205-dirty #2978

[ 1343.104868]  1ffff10036968f44 ffff8801b4b47aa8 ffffffffa23d9a9d ffffffff00000001

[ 1343.104891]  fffffbfff5c2a630 0000000041b58ab3 ffffffffadb3a8f2 ffffffffa23d9905

[ 1343.104914]  0000000000000000 ffff8801b5419b40 fffffbfff7596522 0000000000000001

[ 1343.104919] Call Trace:

[ 1343.104985] dump_stack (lib/dump_stack.c:53)
[ 1343.105060] lockdep_rcu_suspicious (kernel/locking/lockdep.c:4282)
[ 1343.105093] sk_detach_filter (net/core/filter.c:2114 (discriminator 5))
[ 1343.105193] tun_detach_filter (drivers/net/tun.c:1808 (discriminator 7))
[ 1343.105238] __tun_chr_ioctl (drivers/net/tun.c:2133)
[ 1343.105370] tun_chr_ioctl (drivers/net/tun.c:2161)
[ 1343.105407] do_vfs_ioctl (fs/ioctl.c:44 fs/ioctl.c:674)
[ 1343.105506] SyS_ioctl (fs/ioctl.c:689 fs/ioctl.c:680)
[ 1343.105542] entry_SYSCALL_64_fastpath (arch/x86/entry/entry_64.S:200)