fix sk->sk_filter field access

Hello, all!

Function sk_filter() is called from tcp_v{4,6}_rcv() functions with argue needlock = 0, while socket is not locked at that moment. In order to avoid this and similar issues in the future, use rcu for sk->sk_filter field read protection.

Patch is for net-2.6.19

--

 include/linux/filter.h |   13 +++++++------
 include/net/sock.h     |   34 +++++++++++++++++-----------------
 net/core/filter.c      |    8 ++++----
 net/core/sock.c        |   22 +++++++++-------------
 net/dccp/ipv6.c        |    2 +-
 net/decnet/dn_nsp_in.c |    2 +-
 net/ipv4/tcp_ipv4.c    |    2 +-
 net/ipv6/tcp_ipv6.c    |    4 ++--
 net/packet/af_packet.c |   43 ++++++++++++++++++-------------------------
 net/sctp/input.c       |    2 +-
 10 files changed, 61 insertions(+), 71 deletions(-)

index c6cb8f0..91b2e3b 100644
@@ -25,10 +25,10 @@
  
 struct sock_filter	/* Filter block */
 {
-        __u16	code;   /* Actual filter code */
-        __u8	jt;	/* Jump true */
-        __u8	jf;	/* Jump false */
-        __u32	k;      /* Generic multiuse field */
+	__u16	code;   /* Actual filter code */
+	__u8	jt;	/* Jump true */
+	__u8	jf;	/* Jump false */
+	__u32	k;      /* Generic multiuse field */
 };
 
 struct sock_fprog	/* Required for SO_ATTACH_FILTER. */

@@ -41,8 +41,9 @@ struct sock_fprog	/* Required for SO_ATT
 struct sk_filter
 {
 	atomic_t		refcnt;
-        unsigned int         	len;	/* Number of filter blocks */
-        struct sock_filter     	insns[0];
+	unsigned int         	len;	/* Number of filter blocks */
+	struct rcu_head		rcu;
+	struct sock_filter     	insns[0];
 };
 
 static inline unsigned int sk_filter_len(struct sk_filter *fp)

index 337ebec..edd4d73 100644
@@ -862,30 +862,24 @@ extern void sock_init_data(struct socket
  *
  */
 
-static inline int sk_filter(struct sock *sk, struct sk_buff *skb, int 

needlock)
+static inline int sk_filter(struct sock *sk, struct sk_buff *skb)
{
int err;
+ struct sk_filter *filter;

err = security_sock_rcv_skb(sk, skb);
if (err)
return err;

- if (sk->sk_filter) {
- struct sk_filter *filter;
-
- if (needlock)
- bh_lock_sock(sk);
-
- filter = sk->sk_filter;
- if (filter) {
- unsigned int pkt_len = sk_run_filter(skb, filter->insns,
- filter->len);
- err = pkt_len ? pskb_trim(skb, pkt_len) : -EPERM;
- }
-
- if (needlock)
- bh_unlock_sock(sk);
+ rcu_read_lock_bh();
+ filter = sk->sk_filter;
+ if (filter) {
+ unsigned int pkt_len = sk_run_filter(skb, filter->insns,
+ filter->len);
+ err = pkt_len ? pskb_trim(skb, pkt_len) : -EPERM;
}
+ rcu_read_unlock_bh();
+
return err;
}

@@ -897,6 +891,12 @@ static inline int sk_filter(struct sock
* Remove a filter from a socket and release its resources.
*/

+static inline void sk_filter_rcu_free(struct rcu_head *rcu)
+{
+ struct sk_filter *fp = container_of(rcu, struct sk_filter, rcu);
+ kfree(fp);
+}
+
static inline void sk_filter_release(struct sock *sk, struct sk_filter *fp) {
unsigned int size = sk_filter_len(fp);
@@ -904,7 +904,7 @@ static inline void sk_filter_release(str
atomic_sub(size, &sk->sk_omem_alloc);

if (atomic_dec_and_test(&fp->refcnt))
- kfree(fp);
+ call_rcu_bh(&fp->rcu, sk_filter_rcu_free);
}

static inline void sk_filter_charge(struct sock *sk, struct sk_filter *fp)

index 5b4486a..6732782 100644
@@ -422,10 +422,10 @@ int sk_attach_filter(struct sock_fprog *
 	if (!err) {
 		struct sk_filter *old_fp;
 
-		spin_lock_bh(&sk->sk_lock.slock);
-		old_fp = sk->sk_filter;
-		sk->sk_filter = fp;
-		spin_unlock_bh(&sk->sk_lock.slock);
+		rcu_read_lock_bh();
+		old_fp = rcu_dereference(sk->sk_filter);
+		rcu_assign_pointer(sk->sk_filter, fp);
+		rcu_read_unlock_bh();
 		fp = old_fp;
 	}
 

index cfaf090..b77e155 100644
@@ -247,11 +247,7 @@ int sock_queue_rcv_skb(struct sock *sk, 
 		goto out;
 	}
 
-	/* It would be deadlock, if sock_queue_rcv_skb is used
-	   with socket lock! We assume that users of this
-	   function are lock free.
-	*/
-	err = sk_filter(sk, skb, 1);
+	err = sk_filter(sk, skb);
 	if (err)
 		goto out;
 

@@ -278,7 +274,7 @@ int sk_receive_skb(struct sock *sk, stru
 {
 	int rc = NET_RX_SUCCESS;
 
-	if (sk_filter(sk, skb, 0))
+	if (sk_filter(sk, skb))
 		goto discard_and_relse;
 
 	skb->dev = NULL;

@@ -606,15 +602,15 @@ set_rcvbuf:
 			break;
 
 		case SO_DETACH_FILTER:
-			spin_lock_bh(&sk->sk_lock.slock);
-			filter = sk->sk_filter;
+			rcu_read_lock_bh();
+			filter = rcu_dereference(sk->sk_filter);
                         if (filter) {
-				sk->sk_filter = NULL;
-				spin_unlock_bh(&sk->sk_lock.slock);
+				rcu_assign_pointer(sk->sk_filter, NULL);
 				sk_filter_release(sk, filter);
+				rcu_read_unlock_bh();
 				break;
 			}
-			spin_unlock_bh(&sk->sk_lock.slock);
+			rcu_read_unlock_bh();
 			ret = -ENONET;
 			break;
 

@@ -884,10 +880,10 @@ void sk_free(struct sock *sk)
 	if (sk->sk_destruct)
 		sk->sk_destruct(sk);
 
-	filter = sk->sk_filter;
+	filter = rcu_dereference(sk->sk_filter);
 	if (filter) {
 		sk_filter_release(sk, filter);
-		sk->sk_filter = NULL;
+		rcu_assign_pointer(sk->sk_filter, NULL);
 	}
 
 	sock_disable_timestamp(sk);

index f9c5e12..7a47399 100644
@@ -970,7 +970,7 @@ static int dccp_v6_do_rcv(struct sock *s
 	if (skb->protocol == htons(ETH_P_IP))
 		return dccp_v4_do_rcv(sk, skb);
 
-	if (sk_filter(sk, skb, 0))
+	if (sk_filter(sk, skb))
 		goto discard;
 
 	/*

index 86f7f3b..72ecc6e 100644
@@ -586,7 +586,7 @@ static __inline__ int dn_queue_skb(struc
         	goto out;
         }
 
-	err = sk_filter(sk, skb, 0);
+	err = sk_filter(sk, skb);
 	if (err)
 		goto out;
 

index 23b46e3..39b1798 100644
@@ -1104,7 +1104,7 @@ process:
 		goto discard_and_relse;
 	nf_reset(skb);
 
-	if (sk_filter(sk, skb, 0))
+	if (sk_filter(sk, skb))
 		goto discard_and_relse;
 
 	skb->dev = NULL;

index 2b18918..2546fc9 100644
@@ -1075,7 +1075,7 @@ static int tcp_v6_do_rcv(struct sock *sk
 	if (skb->protocol == htons(ETH_P_IP))
 		return tcp_v4_do_rcv(sk, skb);
 
-	if (sk_filter(sk, skb, 0))
+	if (sk_filter(sk, skb))
 		goto discard;
 
 	/*

@@ -1232,7 +1232,7 @@ process:
 	if (!xfrm6_policy_check(sk, XFRM_POLICY_IN, skb))
 		goto discard_and_relse;
 
-	if (sk_filter(sk, skb, 0))
+	if (sk_filter(sk, skb))
 		goto discard_and_relse;
 
 	skb->dev = NULL;

index 0492e83..170bf76 100644
@@ -427,21 +427,24 @@ out_unlock:
 }
 #endif
 
-static inline unsigned run_filter(struct sk_buff *skb, struct sock *sk, 

unsigned res)
+static inline int run_filter(struct sk_buff *skb, struct sock *sk,
+ unsigned *snaplen)
{
struct sk_filter *filter;
+ int err = 0;

- bh_lock_sock(sk);
- filter = sk->sk_filter;
- /*
- * Our caller already checked that filter != NULL but we need to
- * verify that under bh_lock_sock() to be safe
- */
- if (likely(filter != NULL))
- res = sk_run_filter(skb, filter->insns, filter->len);
- bh_unlock_sock(sk);
+ rcu_read_lock_bh();
+ filter = rcu_dereference(sk->sk_filter);
+ if (filter != NULL) {
+ err = sk_run_filter(skb, filter->insns, filter->len);
+ if (!err)
+ err = -EPERM;
+ else if (*snaplen > err)
+ *snaplen = err;
+ }
+ rcu_read_unlock_bh();

- return res;
+ return err;
}

/*
@@ -491,13 +494,8 @@ static int packet_rcv(struct sk_buff *sk

snaplen = skb->len;

- if (sk->sk_filter) {
- unsigned res = run_filter(skb, sk, snaplen);
- if (res == 0)
- goto drop_n_restore;
- if (snaplen > res)
- snaplen = res;
- }
+ if (run_filter(skb, sk, &snaplen) < 0)
+ goto drop_n_restore;

if (atomic_read(&sk->sk_rmem_alloc) + skb->truesize >=
(unsigned)sk->sk_rcvbuf)
@@ -593,13 +591,8 @@ static int tpacket_rcv(struct sk_buff *s

snaplen = skb->len;

- if (sk->sk_filter) {
- unsigned res = run_filter(skb, sk, snaplen);
- if (res == 0)
- goto drop_n_restore;
- if (snaplen > res)
- snaplen = res;
- }
+ if (run_filter(skb, sk, &snaplen) < 0)
+ goto drop_n_restore;

if (sk->sk_type == SOCK_DGRAM) {
macoff = netoff = TPACKET_ALIGN(TPACKET_HDRLEN) + 16;

index 8a34d95..03f65de 100644
@@ -228,7 +228,7 @@ int sctp_rcv(struct sk_buff *skb)
 		goto discard_release;
 	nf_reset(skb);
 
-	if (sk_filter(sk, skb, 1))
+	if (sk_filter(sk, skb))
                 goto discard_release;
 
 	/* Create an SCTP packet structure. */

From: Dmitry Mishin <[email protected]>
Date: Wed, 30 Aug 2006 17:07:14 +0400

What bug are you fixing? The current code looks correct to me.

Why is it important to avoid the needlock stuff in the future?

This patch description is very incomplete, you should list the precise reason why you are doing something instead of using vague language.

Thanks.

Hello!

Yes, explanation could be better.

Current code in tcp_v4_rcv() calls sk_filter() _before_ it takes socket lock. This happened when LSM patches were applied. Apparently, LSM does not want to see socket locked in security_sock_rcv_skb().

Obvious solution is to change the third argument of sk_filter "needlock" to 1. Then we see that sk_filter() is not used with needlock=0 anymore, therefore it can be completely eliminated. It was original fix.

I suggested to remove ugly misuse of bh_lock_sock() (introduced by me, just because there was no better lock to use) and replace it with RCU, which is logical and clean.

The patch looks decent. I had one doubt about misuse of rcu_read_lock_bh() in sk_attach_filter(). Probably, it should be plain local_bh_disable(), I do not know. But because rcu_read_lock_bh() actually is local_bh_disable(), it seems to be not a serious issue.

Alexey

From: Alexey Kuznetsov <[email protected]>
Date: Thu, 31 Aug 2006 02:20:42 +0400

Ok.

Really?

It is used with needlock=0 by DCCP ipv6, for example. This case seems correct too. What about sk_receive_skb()? dn_queue_skb()? In fact, there seems to be numerous uses still with needlock=0, all legitimate.

Let us to fix bugs first, and then consider rewriting the locking. :-)

Hello!

Well, not quite legitime.

sk_receive_skb() has the same bug as tcp_v4_rcv().

Call in backlog processing of ipv6/dccp is occasionally forgotten duplicate of one done in sk_receive_skb().

Yes, those places really added a bit to desire to get rid of this misuse of bh_lock_sock(). :-)

To be honest, I think switching to RCU is the simplest bug fix. At least, it does not require to search where bh_lock_sock() is taken dn_queue_skb(), if it is not forgotten at all. :-)

Alexey

From: Alexey Kuznetsov <[email protected]>
Date: Thu, 31 Aug 2006 03:14:06 +0400

Ok :-)

It is just that, I will have a hard time convincing people that this is legitimate for 2.6.18, and we would like the bug to be fixed there.

From: Dmitry Mishin <[email protected]>
Date: Wed, 30 Aug 2006 17:07:14 +0400

Dmitry, your email client chopped up the longer lines in the patch such as:

Here.

And there.

But I fixed these up and applied the patch to net-2.6.19

I think we seriously need to think about fixing this bug, in so me form, for 2.6.18