Subject: Re: [PATCH -next v2] unix stream: Fix use-after-free crashes

From: "Yan, Zheng" <[email-masked]>
Date: Thu, 08 Sep 2011 15:23:29 +0800

On 09/08/2011 03:11 PM, Eric Dumazet wrote:
> Le jeudi 08 septembre 2011 à 14:22 +0800, Yan, Zheng a écrit :
> 
>> I don't think so. unix_scm_to_skb() calls unix_attach_fds(), it
>> always duplicates scm->fp. 
> 
> What a mess. This code is a nightmare.
> 
> Part of the mess comes from scm_destroy() and scm_release() duplication.
> 
> We should have scm_destroy() only, as before, and NULLify scm->pid/cred
> in unix_scm_to_skb() when we steal references.
> 
> It makes more sense and keeps things clearer.
> 
> 
>  include/net/scm.h  |    9 ---------
>  net/unix/af_unix.c |   27 +++++++++++++++------------
>  2 files changed, 15 insertions(+), 21 deletions(-)
> 
> 
> diff --git a/include/net/scm.h b/include/net/scm.h
> index 68e1e48..2a5b42f 100644
> --- a/include/net/scm.h
> +++ b/include/net/scm.h
> @@ -78,15 +78,6 @@ static __inline__ void scm_destroy(struct scm_cookie *scm)
>  		__scm_destroy(scm);
>  }
>  
> -static __inline__ void scm_release(struct scm_cookie *scm)
> -{
> -	/* keep ref on pid and cred */
> -	scm->pid = NULL;
> -	scm->cred = NULL;
> -	if (scm->fp)
> -		__scm_destroy(scm);
> -}
> -
>  static __inline__ int scm_send(struct socket *sock, struct msghdr *msg,
>  			       struct scm_cookie *scm)
>  {
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index e6d9d10..1fa270a 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -1379,15 +1379,18 @@ static int unix_attach_fds(struct scm_cookie *scm, struct sk_buff *skb)
>  }
>  
>  static int unix_scm_to_skb(struct scm_cookie *scm, struct sk_buff *skb,
> -			   bool send_fds, bool ref)
> +			   bool send_fds, bool steal_refs)
>  {
>  	int err = 0;
> -	if (ref) {
> +
> +	if (!steal_refs) {
>  		UNIXCB(skb).pid  = get_pid(scm->pid);
>  		UNIXCB(skb).cred = get_cred(scm->cred);
>  	} else {
>  		UNIXCB(skb).pid  = scm->pid;
>  		UNIXCB(skb).cred = scm->cred;
> +		scm->pid = NULL;
> +		scm->cred = NULL;
>  	}
>  	UNIXCB(skb).fp = NULL;
>  	if (scm->fp && send_fds)
> @@ -1454,7 +1457,7 @@ static int unix_dgram_sendmsg(struct kiocb *kiocb, struct socket *sock,
>  	if (skb == NULL)
>  		goto out;
>  
> -	err = unix_scm_to_skb(siocb->scm, skb, true, false);
> +	err = unix_scm_to_skb(siocb->scm, skb, true, true);
>  	if (err < 0)
>  		goto out_free;
>  	max_level = err + 1;
> @@ -1550,7 +1553,7 @@ restart:
>  	unix_state_unlock(other);
>  	other->sk_data_ready(other, len);
>  	sock_put(other);
> -	scm_release(siocb->scm);
> +	scm_destroy(siocb->scm);
>  	return len;
>  
>  out_unlock:
> @@ -1577,6 +1580,7 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
>  	int sent = 0;
>  	struct scm_cookie tmp_scm;
>  	bool fds_sent = false;
> +	bool steal_refs = false;
>  	int max_level;
>  
>  	if (NULL == siocb->scm)
> @@ -1638,8 +1642,11 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
>  		size = min_t(int, size, skb_tailroom(skb));
>  
>  
> -		/* Only send the fds and no ref to pid in the first buffer */
> -		err = unix_scm_to_skb(siocb->scm, skb, !fds_sent, fds_sent);
> +		/* Only send the fds in first buffer
> +		 * Last buffer can steal our references to pid/cred
> +		 */
> +		steal_refs = (sent + size >= len);
> +		err = unix_scm_to_skb(siocb->scm, skb, !fds_sent, steal_refs);
>  		if (err < 0) {
>  			kfree_skb(skb);
>  			goto out;
> @@ -1667,10 +1674,7 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
>  		sent += size;
>  	}
>  
> -	if (skb)
> -		scm_release(siocb->scm);
> -	else
> -		scm_destroy(siocb->scm);
> +	scm_destroy(siocb->scm);
>  	siocb->scm = NULL;
>  
>  	return sent;
> @@ -1683,8 +1687,7 @@ pipe_err:
>  		send_sig(SIGPIPE, current, 0);
>  	err = -EPIPE;
>  out_err:
> -	if (skb == NULL)
> -		scm_destroy(siocb->scm);
> +	scm_destroy(siocb->scm);
>  out:
>  	siocb->scm = NULL;
>  	return sent ? : err;
> 
> 

This code looks great, except "goto out;" is still there. I think we should replace it with "goto out_err;" :)

Regards