Date:	Thu, 8 Sep 2011 11:59:27 +0200
From:	Sedat Dilek <sedat.dilek@...glemail.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"Yan, Zheng" <zheng.z.yan@...el.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	"Yan, Zheng" <yanzheng@...n.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"sfr@...b.auug.org.au" <sfr@...b.auug.org.au>,
	"jirislaby@...il.com" <jirislaby@...il.com>,
	"Shi, Alex" <alex.shi@...el.com>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>
Subject: Re: [PATCH -next v2] unix stream: Fix use-after-free crashes

On Thu, Sep 8, 2011 at 9:33 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le jeudi 08 septembre 2011 à 15:23 +0800, Yan, Zheng a écrit :
>
>> This code looks great, except "goto out;" is still there. I think we should replace it with "goto out_err;" :)
>>
>
> Indeed, you're right, thanks
>
>  include/net/scm.h  |    9 ---------
>  net/unix/af_unix.c |   32 +++++++++++++++++---------------
>  2 files changed, 17 insertions(+), 24 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..c8a08ba 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)
> @@ -1638,11 +1642,14 @@ 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;
> +                       goto out_err;
>                }
>                max_level = err + 1;
>                fds_sent = true;
> @@ -1650,7 +1657,7 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
>                err = memcpy_fromiovec(skb_put(skb, size), msg->msg_iov, size);
>                if (err) {
>                        kfree_skb(skb);
> -                       goto out;
> +                       goto out_err;
>                }
>
>                unix_state_lock(other);
> @@ -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;
> @@ -1683,9 +1687,7 @@ pipe_err:
>                send_sig(SIGPIPE, current, 0);
>        err = -EPIPE;
>  out_err:
> -       if (skb == NULL)
> -               scm_destroy(siocb->scm);
> -out:
> +       scm_destroy(siocb->scm);
>        siocb->scm = NULL;
>        return sent ? : err;
>  }
>

I have tested this fixup patch on i386.
Can we have a separate patch with corrected descriptive text?

Thanks to all involved people.

- Sedat -

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