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

From: Daniel Borkmann <[email-masked]>
Date: Tue, 29 Mar 2016 16:39:43 +0200

On 03/29/2016 03:55 PM, Daniel Borkmann wrote:
> [ dropping my old email address ]
>
> On 03/29/2016 02:58 PM, Michal Kubecek wrote:
>> 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.
>
> Wrt return value, afaik SOCK_FILTER_LOCKED cannot be set for tun devs, so we
> should be okay.
>
>> 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;
>>
>
> In tun case, the control path for tun_attach_filter() and tun_detach_filter()
> is under RTNL lock (held in __tun_chr_ioctl()).
>
> So in the BPF core the rcu_dereference_protected(<sk_filter>, sock_owned_by_user(sk))
> looks like a false positive in this specific use case to me, that we should probably
> just silence.
>
> Running the filter via sk_filter() in tun device happens under rcu_read_lock(),
> so the dereference and assignment pair seems okay to me.
>
> Was wondering whether we should convert this to unattached BPF filter, but this
> would break with existing expectations from sk_filter() (e.g. security modules).

If we want to silence it, could be something like the below (only compile-tested):

  drivers/net/tun.c      |  8 +++++---
  include/linux/filter.h |  4 ++++
  net/core/filter.c      | 33 +++++++++++++++++++++------------
  3 files changed, 30 insertions(+), 15 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index afdf950..510e90a 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -622,7 +622,8 @@ 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;
  	}
@@ -1822,7 +1823,7 @@ 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;
@@ -1835,7 +1836,8 @@ 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;
diff --git a/include/linux/filter.h b/include/linux/filter.h
index 43aa1f8..a51a536 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -465,10 +465,14 @@ int bpf_prog_create_from_user(struct bpf_prog **pfp, struct sock_fprog *fprog,
  void bpf_prog_destroy(struct bpf_prog *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_attach_bpf(u32 ufd, struct sock *sk);
  int sk_reuseport_attach_filter(struct sock_fprog *fprog, struct sock *sk);
  int sk_reuseport_attach_bpf(u32 ufd, struct sock *sk);
  int sk_detach_filter(struct sock *sk);
+int __sk_detach_filter(struct sock *sk, bool locked);
+
  int sk_get_filter(struct sock *sk, struct sock_filter __user *filter,
  		  unsigned int len);

diff --git a/net/core/filter.c b/net/core/filter.c
index 2429918..02f2f6c 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -1149,7 +1149,8 @@ void bpf_prog_destroy(struct bpf_prog *fp)
  }
  EXPORT_SYMBOL_GPL(bpf_prog_destroy);

-static int __sk_attach_prog(struct bpf_prog *prog, struct sock *sk)
+static int __sk_attach_prog(struct bpf_prog *prog, struct sock *sk,
+			    bool locked)
  {
  	struct sk_filter *fp, *old_fp;

@@ -1165,10 +1166,8 @@ static int __sk_attach_prog(struct bpf_prog *prog, struct sock *sk)
  		return -ENOMEM;
  	}

-	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)
  		sk_filter_uncharge(sk, old_fp);

@@ -1247,7 +1246,8 @@ struct bpf_prog *__get_filter(struct sock_fprog *fprog, struct sock *sk)
   * 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 bpf_prog *prog = __get_filter(fprog, sk);
  	int err;
@@ -1255,7 +1255,7 @@ int sk_attach_filter(struct sock_fprog *fprog, struct sock *sk)
  	if (IS_ERR(prog))
  		return PTR_ERR(prog);

-	err = __sk_attach_prog(prog, sk);
+	err = __sk_attach_prog(prog, sk, locked);
  	if (err < 0) {
  		__bpf_prog_release(prog);
  		return err;
@@ -1263,7 +1263,12 @@ 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_attach_filter(struct sock_fprog *fprog, struct sock *sk)
+{
+	return __sk_attach_filter(fprog, sk, sock_owned_by_user(sk));
+}

  int sk_reuseport_attach_filter(struct sock_fprog *fprog, struct sock *sk)
  {
@@ -1309,7 +1314,7 @@ int sk_attach_bpf(u32 ufd, struct sock *sk)
  	if (IS_ERR(prog))
  		return PTR_ERR(prog);

-	err = __sk_attach_prog(prog, sk);
+	err = __sk_attach_prog(prog, sk, sock_owned_by_user(sk));
  	if (err < 0) {
  		bpf_prog_put(prog);
  		return err;
@@ -2445,7 +2450,7 @@ static int __init register_sk_filter_ops(void)
  }
  late_initcall(register_sk_filter_ops);

-int sk_detach_filter(struct sock *sk)
+int __sk_detach_filter(struct sock *sk, bool locked)
  {
  	int ret = -ENOENT;
  	struct sk_filter *filter;
@@ -2453,8 +2458,7 @@ 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);
@@ -2463,7 +2467,12 @@ 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)
-- 
1.9.3