Subject: Re: [PATCH] net: Fix sock_wfree() race

From: Eric Dumazet <[email-masked]>
Date: Thu, 24 Sep 2009 22:49:24 +0200

Jarek Poplawski a écrit :
> Eric Dumazet wrote, On 09/23/2009 03:44 PM:
> 
> ...
>> Here is the patch for reference :
>>
>> [PATCH] net: Fix sock_wfree() race
>>
>> Commit 2b85a34e911bf483c27cfdd124aeb1605145dc80
>> (net: No more expensive sock_hold()/sock_put() on each tx)
>> opens a window in sock_wfree() where another cpu
>> might free the socket we are working on.
>>
>> A fix is to call sk->sk_write_space(sk) while still
>> holding a reference on sk.
>>
>>
>> Reported-by: Jike Song <[email-masked]>
>> Signed-off-by: Eric Dumazet <[email-masked]>
>> ---
>>  net/core/sock.c |   19 ++++++++++++-------
>>  1 files changed, 12 insertions(+), 7 deletions(-)
>>
>> diff --git a/net/core/sock.c b/net/core/sock.c
>> index 30d5446..e1f034e 100644
>> --- a/net/core/sock.c
>> +++ b/net/core/sock.c
>> @@ -1228,17 +1228,22 @@ void __init sk_init(void)
>>  void sock_wfree(struct sk_buff *skb)
>>  {
>>  	struct sock *sk = skb->sk;
>> -	int res;
>> +	unsigned int len = skb->truesize;
>>  
>> -	/* In case it might be waiting for more memory. */
>> -	res = atomic_sub_return(skb->truesize, &sk->sk_wmem_alloc);
>> -	if (!sock_flag(sk, SOCK_USE_WRITE_QUEUE))
>> +	if (!sock_flag(sk, SOCK_USE_WRITE_QUEUE)) {
>> +		/*
>> +		 * Keep a reference on sk_wmem_alloc, this will be released
>> +		 * after sk_write_space() call
>> +		 */
>> +		atomic_sub(len - 1, &sk->sk_wmem_alloc);
>>  		sk->sk_write_space(sk);
>> +		len = 1;
>> +	}
>>  	/*
>> -	 * if sk_wmem_alloc reached 0, we are last user and should
>> -	 * free this sock, as sk_free() call could not do it.
>> +	 * if sk_wmem_alloc reaches 0, we must finish what sk_free()
>> +	 * could not do because of in-flight packets
>>  	 */
>> -	if (res == 0)
>> +	if (atomic_sub_return(len, &sk->sk_wmem_alloc) == 0)
>>  		__sk_free(sk);
> 
> 
> Probably atomic_sub_and_test() is more popular for this.

Indeed, as x86 can generate faster code (no need of xadd instruction)

Thanks Jarek

[PATCH] net: Fix sock_wfree() race

Commit 2b85a34e911bf483c27cfdd124aeb1605145dc80
(net: No more expensive sock_hold()/sock_put() on each tx)
opens a window in sock_wfree() where another cpu
might free the socket we are working on.

A fix is to call sk->sk_write_space(sk) while still
holding a reference on sk.

Reported-by: Jike Song <[email-masked]>
Signed-off-by: Eric Dumazet <[email-masked]>
---
 net/core/sock.c |   19 ++++++++++++-------
 1 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/net/core/sock.c b/net/core/sock.c
index 30d5446..e1f034e 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1228,17 +1228,22 @@ void __init sk_init(void)
 void sock_wfree(struct sk_buff *skb)
 {
 	struct sock *sk = skb->sk;
-	int res;
+	unsigned int len = skb->truesize;
 
-	/* In case it might be waiting for more memory. */
-	res = atomic_sub_return(skb->truesize, &sk->sk_wmem_alloc);
-	if (!sock_flag(sk, SOCK_USE_WRITE_QUEUE))
+	if (!sock_flag(sk, SOCK_USE_WRITE_QUEUE)) {
+		/*
+		 * Keep a reference on sk_wmem_alloc, this will be released
+		 * after sk_write_space() call
+		 */
+		atomic_sub(len - 1, &sk->sk_wmem_alloc);
 		sk->sk_write_space(sk);
+		len = 1;
+	}
 	/*
-	 * if sk_wmem_alloc reached 0, we are last user and should
-	 * free this sock, as sk_free() call could not do it.
+	 * if sk_wmem_alloc reaches 0, we must finish what sk_free()
+	 * could not do because of in-flight packets
 	 */
-	if (res == 0)
+	if (atomic_sub_and_test(len, &sk->sk_wmem_alloc))
 		__sk_free(sk);
 }
 EXPORT_SYMBOL(sock_wfree);