ipv4: dst_entry leak in ip_append_data()

PATCH In review Vasily Averin 3 participants 7 messages Oct 14, 2014 – Oct 15, 2014

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Networking Network drivers IPv4/IPv6

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Vasily Averin

author

2014-10-14 04:58 UTC #0 lore raw

PATCH v2 1/1 ipv4: dst_entry leak in ip_append_data()

v2: adjust the indentation of the arguments __ip_append_data() call

Fixes 2e77d89b2fa8 ("net: avoid a pair of dst_hold()/dst_release() in ip_append_data()")

If sk_write_queue is empty ip_append_data() executes ip_setup_cork() that "steals" dst entry from rt to cork. Later it calls __ip_append_data() that creates skb and adds it to sk_write_queue.

If skb was added successfully following ip_push_pending_frames() call reassign dst entries from cork to skb, and kfree_skb frees dst_entry.

However nobody frees stolen dst_entry if skb was not added into sk_write_queue.

Signed-off-by Vasily Averin <[email protected]>

---
 net/ipv4/ip_output.c | 29 +++++++++++++++++------------
 1 file changed, 17 insertions(+), 12 deletions(-)

net/ipv4/ip_output.c +17 ~~-12~~

index e35b712..3ba2291 100644
@@ -1120,6 +1120,15 @@ static int ip_setup_cork(struct sock *sk, struct inet_cork *cork,
 	return 0;
 }
 
+static void ip_cork_release(struct inet_cork *cork)
+{
+	cork->flags &= ~IPCORK_OPT;
+	kfree(cork->opt);
+	cork->opt = NULL;
+	dst_release(cork->dst);
+	cork->dst = NULL;
+}
+
 /*
  *	ip_append_data() and ip_append_page() can make one large IP datagram
  *	from many pieces of data. Each pieces will be holded on the socket

@@ -1152,9 +1161,14 @@ int ip_append_data(struct sock *sk, struct flowi4 *fl4,
 		transhdrlen = 0;
 	}
 
-	return __ip_append_data(sk, fl4, &sk->sk_write_queue, &inet->cork.base,
-				sk_page_frag(sk), getfrag,
-				from, length, transhdrlen, flags);
+	err = __ip_append_data(sk, fl4, &sk->sk_write_queue, &inet->cork.base,
+			       sk_page_frag(sk), getfrag,
+			       from, length, transhdrlen, flags);
+
+	if (skb_queue_empty(&sk->sk_write_queue))
+		ip_cork_release(&inet->cork.base);
+
+	return err;
 }
 
 ssize_t	ip_append_page(struct sock *sk, struct flowi4 *fl4, struct page *page,

@@ -1304,15 +1318,6 @@ error:
 	return err;
 }
 
-static void ip_cork_release(struct inet_cork *cork)
-{
-	cork->flags &= ~IPCORK_OPT;
-	kfree(cork->opt);
-	cork->opt = NULL;
-	dst_release(cork->dst);
-	cork->dst = NULL;
-}
-
 /*
  *	Combined all pending IP fragments on the socket as one IP datagram
  *	and push them out.

Signature · 1 line

1.9.1

David Miller

maintainer

2014-10-14 20:12 UTC #1 lore raw

From: Vasily Averin <[email protected]>
Date: Tue, 14 Oct 2014 08:57:14 +0400

Quoted 8 lines

v2: adjust the indentation of the arguments __ip_append_data() call
Fixes 2e77d89b2fa8 ("net: avoid a pair of dst_hold()/dst_release() in ip_append_data()")
If sk_write_queue is empty ip_append_data() executes ip_setup_cork() that "steals" dst entry from rt to cork. Later it calls __ip_append_data() that creates skb and adds it to sk_write_queue.

Quoted 6 lines

If skb was added successfully following ip_push_pending_frames() call reassign dst entries from cork to skb, and kfree_skb frees dst_entry.
However nobody frees stolen dst_entry if skb was not added into sk_write_queue.
Signed-off-by Vasily Averin <[email protected]>

Why doesn't ip_make_skb() need the same fix? It seems to do the same thing.

Vasily Averin

author

2014-10-15 07:49 UTC #2 lore raw

Quoted 14 lines · David Miller

From: Vasily Averin <[email protected]>
Date: Tue, 14 Oct 2014 08:57:14 +0400
Quoted 11 lines
v2: adjust the indentation of the arguments __ip_append_data() call
Fixes 2e77d89b2fa8 ("net: avoid a pair of dst_hold()/dst_release() in ip_append_data()")
If sk_write_queue is empty ip_append_data() executes ip_setup_cork() that "steals" dst entry from rt to cork. Later it calls __ip_append_data() that creates skb and adds it to sk_write_queue.
If skb was added successfully following ip_push_pending_frames() call reassign dst entries from cork to skb, and kfree_skb frees dst_entry.

Quoted 6 lines

Quoted 3 lines
However nobody frees stolen dst_entry if skb was not added into sk_write_queue.
Signed-off-by Vasily Averin <[email protected]>
Why doesn't ip_make_skb() need the same fix? It seems to do the same thing.

It seems for me ip_make_skb() works (almost) correctly, but seems refcounting can be is incorrect if queue can be not empty (Please see details below).

If __ip_append_data() returns errors ip_make_skb() calls __ip_flush_pending_frames() that calls ip_cork_release() inside and frees stolen dst_entry.

If __ip_append_data() returns success -- dst refcounter changes are not required. In this case skb will be created and added to queue (and it will not be empty) Later in __ip_make_skb() these skb will get dst reference,
and refcounter will be decremented during kfree_skb().

I do not like that there is such unclear dependency between functions, but seems currently it works correctly.

However I afraid dst refcountng can work incorrectly if sk_write_queue can be not empty at the moment of ip_append_data() call.
It was not happen in case ip_send_unicast_reply() but probably can happen in other places.

Let's calculate dst refcounters changes in this case.

First packet:

dst_refcount increment was happen in ip_append_data() caller, taken during rt lookup
- ip_append_data():
-- sk_write_queue is empty, ip_setup_cork() steals dst entry
-- __ip_append_data() adds skb to queue, queue is not flushed, waiting for next packets. ip_rt_put in ip_append_data() caller does not work, because dst reference was stolen.

dst refcount here +1

then we want to sent 2nd packet:
dst refcount increment was happen in ip_append_data() caller
- ip_append_data():
-- sk_write_queue is NOT empty, dst was not stolen
-- __ip_append_data() adds skb to queue
ip_rt_put in ip_append_data() caller decrements dst refcount, because it as not stolen

dst refcount here +1

Then we handle new packets, all of them are added to queue

dst refcount is still +1

Then queue is flushed.
Each packet in queue get dst reference from cork,
Each kfree_skb decrements dst refcounter, and it may become negative.

Am I wrong probably?

Eric Dumazet

2014-10-15 04:46 UTC #3 lore raw

In reply to Vasily Averin

Quoted 4 lines · Vasily Averin

v2: adjust the indentation of the arguments __ip_append_data() call
Fixes 2e77d89b2fa8 ("net: avoid a pair of dst_hold()/dst_release() in ip_append_data()")

Quoted 8 lines

If sk_write_queue is empty ip_append_data() executes ip_setup_cork() that "steals" dst entry from rt to cork. Later it calls __ip_append_data() that creates skb and adds it to sk_write_queue.
If skb was added successfully following ip_push_pending_frames() call reassign dst entries from cork to skb, and kfree_skb frees dst_entry.
However nobody frees stolen dst_entry if skb was not added into sk_write_queue.

I thought this was done by ip_flush_pending_frames() ?

Can you describe the issue more precisely ?

Thanks !

Vasily Averin

author

2014-10-15 06:58 UTC #4 lore raw

Quoted 9 lines · Eric Dumazet

Quoted 8 lines · Vasily Averin
v2: adjust the indentation of the arguments __ip_append_data() call
Fixes 2e77d89b2fa8 ("net: avoid a pair of dst_hold()/dst_release() in ip_append_data()")
If sk_write_queue is empty ip_append_data() executes ip_setup_cork() that "steals" dst entry from rt to cork. Later it calls __ip_append_data() that creates skb and adds it to sk_write_queue.

Quoted 6 lines

Quoted 4 lines
If skb was added successfully following ip_push_pending_frames() call reassign dst entries from cork to skb, and kfree_skb frees dst_entry.
However nobody frees stolen dst_entry if skb was not added into sk_write_queue.
I thought this was done by ip_flush_pending_frames() ?

Take look at ip_send_unicast_reply():

ip_flush_pending_frames() is not called if skb was not added to sk_write_queue. And ip_rt_put() does not work, because dst entry was stolen in ip_setup_cork().

Probably it can happen in raw_sendmsg() and udp_sendmsg() too.

Eric Dumazet

2014-10-15 09:30 UTC #5 lore raw

Quoted 10 lines · Vasily Averin

Quoted 9 lines · Eric Dumazet
Quoted 8 lines · Vasily Averin
v2: adjust the indentation of the arguments __ip_append_data() call
Fixes 2e77d89b2fa8 ("net: avoid a pair of dst_hold()/dst_release() in ip_append_data()")
If sk_write_queue is empty ip_append_data() executes ip_setup_cork() that "steals" dst entry from rt to cork. Later it calls __ip_append_data() that creates skb and adds it to sk_write_queue.

Quoted 8 lines

Quoted 6 lines
Quoted 4 lines
If skb was added successfully following ip_push_pending_frames() call reassign dst entries from cork to skb, and kfree_skb frees dst_entry.
However nobody frees stolen dst_entry if skb was not added into sk_write_queue.
I thought this was done by ip_flush_pending_frames() ?
Take look at ip_send_unicast_reply():

So maybe the bug is here ?

Quoted 5 lines

ip_flush_pending_frames() is not called if skb was not added to sk_write_queue. And ip_rt_put() does not work, because dst entry was stolen in ip_setup_cork().
Probably it can happen in raw_sendmsg() and udp_sendmsg() too.

UDP & RAW do :

err = ip_append_data(...);
if (err)

    udp_flush_pending_frames(sk);

It seems you chose to add a test in fast path, with not even adding an unlikely() clause, while it seems that we took care of all the cases but missed a single one : ip_send_unicast_reply()

I am suggesting to fix this bug in another way.

Thanks.

Vasily Averin

author

2014-10-15 11:33 UTC #6 lore raw

Quoted 14 lines · Eric Dumazet

Quoted 13 lines · Vasily Averin
Quoted 12 lines · Eric Dumazet
Quoted 11 lines · Vasily Averin
v2: adjust the indentation of the arguments __ip_append_data() call
Fixes 2e77d89b2fa8 ("net: avoid a pair of dst_hold()/dst_release() in ip_append_data()")
If sk_write_queue is empty ip_append_data() executes ip_setup_cork() that "steals" dst entry from rt to cork. Later it calls __ip_append_data() that creates skb and adds it to sk_write_queue.
If skb was added successfully following ip_push_pending_frames() call reassign dst entries from cork to skb, and kfree_skb frees dst_entry.

Quoted 7 lines

Quoted 5 lines
Quoted 3 lines
Quoted 1 line
However nobody frees stolen dst_entry if skb was not added into sk_write_queue.
I thought this was done by ip_flush_pending_frames() ?
Take look at ip_send_unicast_reply():
So maybe the bug is here ?

Thank you, I'll remake my patch.