tun, bpf: fix suspicious RCU usage in tun_{attach, detach}_filter

[ Upstream commit 5a5abb1f ]

Sasha Levin reported a suspicious rcu_dereference_protected() warning
found while fuzzing with trinity that is similar to this one:

  [   52.765684] net/core/filter.c:2262 suspicious rcu_dereference_protected() usage!
  [   52.765688] other info that might help us debug this:
  [   52.765695] rcu_scheduler_active = 1, debug_locks = 1
  [   52.765701] 1 lock held by a.out/1525:
  [   52.765704]  #0:  (rtnl_mutex){+.+.+.}, at: [<ffffffff816a64b7>] rtnl_lock+0x17/0x20
  [   52.765721] stack backtrace:
  [   52.765728] CPU: 1 PID: 1525 Comm: a.out Not tainted 4.5.0+ #264
  [...]
  [   52.765768] Call Trace:
  [   52.765775]  [<ffffffff813e488d>] dump_stack+0x85/0xc8
  [   52.765784]  [<ffffffff810f2fa5>] lockdep_rcu_suspicious+0xd5/0x110
  [   52.765792]  [<ffffffff816afdc2>] sk_detach_filter+0x82/0x90
  [   52.765801]  [<ffffffffa0883425>] tun_detach_filter+0x35/0x90 [tun]
  [   52.765810]  [<ffffffffa0884ed4>] __tun_chr_ioctl+0x354/0x1130 [tun]
  [   52.765818]  [<ffffffff8136fed0>] ? selinux_file_ioctl+0x130/0x210
  [   52.765827]  [<ffffffffa0885ce3>] tun_chr_ioctl+0x13/0x20 [tun]
  [   52.765834]  [<ffffffff81260ea6>] do_vfs_ioctl+0x96/0x690
  [   52.765843]  [<ffffffff81364af3>] ? security_file_ioctl+0x43/0x60
  [   52.765850]  [<ffffffff81261519>] SyS_ioctl+0x79/0x90
  [   52.765858]  [<ffffffff81003ba2>] do_syscall_64+0x62/0x140
  [   52.765866]  [<ffffffff817d563f>] entry_SYSCALL64_slow_path+0x25/0x25

Same can be triggered with PROVE_RCU (+ PROVE_RCU_REPEATEDLY) enabled
from tun_attach_filter() when user space calls ioctl(tun_fd, TUN{ATTACH,
DETACH}FILTER, ...) for adding/removing a BPF filter on tap devices.

Since the fix in f91ff5b9 ("net: sk_{detach|attach}_filter() rcu
fixes") sk_attach_filter()/sk_detach_filter() now dereferences the
filter with rcu_dereference_protected(), checking whether socket lock
is held in control path.

Since its introduction in 99405162 ("tun: socket filter support"),
tap filters are managed under RTNL lock from __tun_chr_ioctl(). Thus the
sock_owned_by_user(sk) doesn't apply in this specific case and therefore
triggers the false positive.

Extend the BPF API with __sk_attach_filter()/__sk_detach_filter() pair
that is used by tap filters and pass in lockdep_rtnl_is_held() for the
rcu_dereference_protected() checks instead.
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.16:
 - Drop changes to sk_attach_bpf(), __sk_attach_prog()
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/net/tun.c

@@ -553,7 +553,8 @@ static int tun_attach(struct tun_struct *tun, struct file *file, bool skip_filte
 
 	/* Re-attach the filter to persist device */
 	if (!skip_filter && (tun->filter_attached == true)) {
-		err = sk_attach_filter(&tun->fprog, tfile->socket.sk);
+		err = __sk_attach_filter(&tun->fprog, tfile->socket.sk,
+					 lockdep_rtnl_is_held());
 		if (!err)
 			goto out;
 	}
@@ -1792,7 +1793,7 @@ static void tun_detach_filter(struct tun_struct *tun, int n)
 
 	for (i = 0; i < n; i++) {
 		tfile = rtnl_dereference(tun->tfiles[i]);
-		sk_detach_filter(tfile->socket.sk);
+		__sk_detach_filter(tfile->socket.sk, lockdep_rtnl_is_held());
 	}
 
 	tun->filter_attached = false;
@@ -1805,7 +1806,8 @@ static int tun_attach_filter(struct tun_struct *tun)
 
 	for (i = 0; i < tun->numqueues; i++) {
 		tfile = rtnl_dereference(tun->tfiles[i]);
-		ret = sk_attach_filter(&tun->fprog, tfile->socket.sk);
+		ret = __sk_attach_filter(&tun->fprog, tfile->socket.sk,
+					 lockdep_rtnl_is_held());
 		if (ret) {
 			tun_detach_filter(tun, i);
 			return ret;

include/linux/filter.h

@@ -359,7 +359,10 @@ int sk_unattached_filter_create(struct sk_filter **pfp,
 void sk_unattached_filter_destroy(struct sk_filter *fp);
 
 int sk_attach_filter(struct sock_fprog *fprog, struct sock *sk);
+int __sk_attach_filter(struct sock_fprog *fprog, struct sock *sk,
+		       bool locked);
 int sk_detach_filter(struct sock *sk);
+int __sk_detach_filter(struct sock *sk, bool locked);
 
 int sk_chk_filter(struct sock_filter *filter, unsigned int flen);
 int sk_get_filter(struct sock *sk, struct sock_filter __user *filter,

net/core/filter.c

@@ -1590,7 +1590,8 @@ EXPORT_SYMBOL_GPL(sk_unattached_filter_destroy);
  * occurs or there is insufficient memory for the filter a negative
  * errno code is returned. On success the return is zero.
  */
-int sk_attach_filter(struct sock_fprog *fprog, struct sock *sk)
+int __sk_attach_filter(struct sock_fprog *fprog, struct sock *sk,
+		       bool locked)
 {
 	struct sk_filter *fp, *old_fp;
 	unsigned int fsize = sk_filter_proglen(fprog);
@@ -1629,8 +1630,7 @@ int sk_attach_filter(struct sock_fprog *fprog, struct sock *sk)
 	if (IS_ERR(fp))
 		return PTR_ERR(fp);
 
-	old_fp = rcu_dereference_protected(sk->sk_filter,
-					   sock_owned_by_user(sk));
+	old_fp = rcu_dereference_protected(sk->sk_filter, locked);
 	rcu_assign_pointer(sk->sk_filter, fp);
 
 	if (old_fp)
@@ -1638,9 +1638,14 @@ int sk_attach_filter(struct sock_fprog *fprog, struct sock *sk)
 
 	return 0;
 }
-EXPORT_SYMBOL_GPL(sk_attach_filter);
+EXPORT_SYMBOL_GPL(__sk_attach_filter);
 
-int sk_detach_filter(struct sock *sk)
+int sk_attach_filter(struct sock_fprog *fprog, struct sock *sk)
+{
+	return __sk_attach_filter(fprog, sk, sock_owned_by_user(sk));
+}
+
+int __sk_detach_filter(struct sock *sk, bool locked)
 {
 	int ret = -ENOENT;
 	struct sk_filter *filter;
@@ -1648,8 +1653,7 @@ int sk_detach_filter(struct sock *sk)
 	if (sock_flag(sk, SOCK_FILTER_LOCKED))
 		return -EPERM;
 
-	filter = rcu_dereference_protected(sk->sk_filter,
-					   sock_owned_by_user(sk));
+	filter = rcu_dereference_protected(sk->sk_filter, locked);
 	if (filter) {
 		RCU_INIT_POINTER(sk->sk_filter, NULL);
 		sk_filter_uncharge(sk, filter);
@@ -1658,7 +1662,12 @@ int sk_detach_filter(struct sock *sk)
 
 	return ret;
 }
-EXPORT_SYMBOL_GPL(sk_detach_filter);
+EXPORT_SYMBOL_GPL(__sk_detach_filter);
+
+int sk_detach_filter(struct sock *sk)
+{
+	return __sk_detach_filter(sk, sock_owned_by_user(sk));
+}
 
 int sk_get_filter(struct sock *sk, struct sock_filter __user *ubuf,
 		  unsigned int len)