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Message-ID: <4E685F19.6030407@intel.com>
Date: Thu, 08 Sep 2011 14:22:17 +0800
From: "Yan, Zheng" <zheng.z.yan@...el.com>
To: Eric Dumazet <eric.dumazet@...il.com>
CC: Tim Chen <tim.c.chen@...ux.intel.com>,
"sedat.dilek@...il.com" <sedat.dilek@...il.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 09/08/2011 01:59 PM, Eric Dumazet wrote:
> Le mercredi 07 septembre 2011 à 14:06 -0700, Tim Chen a écrit :
>> On Thu, 2011-09-08 at 08:27 +0800, Yan, Zheng wrote:
>>
>>>> err = -EPIPE;
>>>> out_err:
>>>> - if (skb == NULL)
>>>> + if (!steal_refs)
>>>> scm_destroy(siocb->scm);
>>>
>>> I think we should call scm_release() here in the case of
>>> steal_refs == true. Otherwise siocb->scm->fp may leak.
>>
>> Yan Zheng,
>>
>> I've updated the patch. If it looks good to you now, can you add your
>> Signed-off-by to the patch. Pending Sedat's testing on this patch,
>> I think it is good to go.
>>
>> Tim
>>
>> ---
>> Commit 0856a30409 (Scm: Remove unnecessary pid & credential references
>> in Unix socket's send and receive path) introduced a use-after-free bug.
>> The sent skbs from unix_stream_sendmsg could be consumed and destructed
>> by the receive side, removing all references to the credentials,
>> before the send side has finished sending out all
>> packets. However, send side could continue to consturct new packets in the
>> stream, using credentials that have lost its last reference and been
>> freed.
>>
>> In this fix, we don't steal the reference to credentials we have obtained
>> in scm_send at beginning of unix_stream_sendmsg, till we've reached
>> the last packet. This fixes the problem in commit 0856a30409.
>>
>> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
>> Reported-by: Jiri Slaby <jirislaby@...il.com>
>> Tested-by: Sedat Dilek <sedat.dilek@...glemail.com>
>> Tested-by: Valdis Kletnieks <Valdis.Kletnieks@...edu>
>> ---
>> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
>> index 136298c..47780dc 100644
>> --- a/net/unix/af_unix.c
>> +++ b/net/unix/af_unix.c
>> @@ -1383,10 +1383,11 @@ 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 {
>> @@ -1458,7 +1459,7 @@ 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;
>> @@ -1581,6 +1582,7 @@ 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)
>> @@ -1642,11 +1644,14 @@ 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;
>> @@ -1654,7 +1659,7 @@ 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);
>> @@ -1671,7 +1676,7 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
>> sent += size;
>> }
>>
>> - if (skb)
>> + if (steal_refs)
>> scm_release(siocb->scm);
>> else
>> scm_destroy(siocb->scm);
>> @@ -1687,9 +1692,10 @@ pipe_err:
>> send_sig(SIGPIPE, current, 0);
>> err = -EPIPE;
>> out_err:
>> - if (skb == NULL)
>> + if (steal_refs)
>> + scm_release(siocb->scm);
>> + else
>> scm_destroy(siocb->scm);
>> -out:
>> siocb->scm = NULL;
>> return sent ? : err;
>> }
>>
>>
>>
>
> I dont think this patch is good.
>
> Sedat traces have nothing to do with af_unix.
>
> Once unix_scm_to_skb() was called and successful, and steal_refs is true
> our refs are attached to this skb. They will be released by
> skb_free(skb). Same for fp : They either were sent in a previous skb or
> this one.
>
> This is why the "goto out;" was OK.
>
I don't think so. unix_scm_to_skb() calls unix_attach_fds(), it
always duplicates scm->fp.
Regards
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