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

[Michal Kubecek]

On Mon, Feb 22, 2016 at 10:31:33AM -0500, Sasha Levin wrote:
> 
> 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)

Looks like sk_detach_filter() wants the socket to be owned which neither
tun_detach_filter() does not do, unlike sock_setsockopt(). Could you
check if the patch below helps?

I'm also not really sure if it is safe to ignore return value of
sk_detach_filter() and just sets tun->filter_attached to false - but
it's not really clear what should be done if one of the calls fails
after some succeeded.


diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index afdf950617c3..7417d7c20bab 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1818,11 +1818,13 @@ static int set_offload(struct tun_struct *tun, unsigned 
long arg)
 static void tun_detach_filter(struct tun_struct *tun, int n)
 {
        int i;
-       struct tun_file *tfile;
 
        for (i = 0; i < n; i++) {
-               tfile = rtnl_dereference(tun->tfiles[i]);
-               sk_detach_filter(tfile->socket.sk);
+               struct sock *sk = rtnl_dereference(tun->tfiles[i])->socket.sk;
+
+               lock_sock(sk);
+               sk_detach_filter(sk);
+               release_sock(sk);
        }
 
        tun->filter_attached = false;