netdev - Re: [PATCH net] sctp: check stream reset info len before making reconf chunk

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Date:   Mon, 13 Nov 2017 10:09:34 -0500
From:   Neil Horman <nhorman@...driver.com>
To:     Xin Long <lucien.xin@...il.com>
Cc:     network dev <netdev@...r.kernel.org>, linux-sctp@...r.kernel.org,
        davem@...emloft.net,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, syzkaller@...glegroups.com
Subject: Re: [PATCH net] sctp: check stream reset info len before making
 reconf chunk

On Mon, Nov 13, 2017 at 01:39:27PM +0800, Xin Long wrote:
> Now when resetting stream, if both in and out flags are set, the info
> len can reach:
>   sizeof(struct sctp_strreset_outreq) + SCTP_MAX_STREAM(65535) +
>   sizeof(struct sctp_strreset_inreq)  + SCTP_MAX_STREAM(65535)
> even without duplicated stream no, this value is far greater than the
> chunk's max size.
> 
> _sctp_make_chunk doesn't do any check for this, which would cause the
> skb it allocs is huge, syzbot even reported a crash due to this.
> 
> This patch is to check stream reset info len before making reconf
> chunk and return NULL if the len exceeds chunk's capacity.
> 
> Fixes: cc16f00f6529 ("sctp: add support for generating stream reconf ssn reset request chunk")
> Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
> Signed-off-by: Xin Long <lucien.xin@...il.com>
> ---
>  net/sctp/sm_make_chunk.c | 7 +++++--
>  net/sctp/stream.c        | 8 +++++---
>  2 files changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/net/sctp/sm_make_chunk.c b/net/sctp/sm_make_chunk.c
> index 514465b..a21328a 100644
> --- a/net/sctp/sm_make_chunk.c
> +++ b/net/sctp/sm_make_chunk.c
> @@ -3598,14 +3598,17 @@ struct sctp_chunk *sctp_make_strreset_req(
>  	__u16 stream_len = stream_num * 2;
>  	struct sctp_strreset_inreq inreq;
>  	struct sctp_chunk *retval;
> -	__u16 outlen, inlen;
> +	int outlen, inlen;
>  
>  	outlen = (sizeof(outreq) + stream_len) * out;
>  	inlen = (sizeof(inreq) + stream_len) * in;
>  
> +	if (outlen + inlen > SCTP_MAX_CHUNK_LEN - sizeof(struct sctp_chunkhdr))
> +		return ERR_PTR(-EINVAL);
> +
Why all the ERR_PTR manipulations here?  Just returning NULL, like the fuction
has been doing is sufficient to set ENOMEM at both call sites

Neil

>  	retval = sctp_make_reconf(asoc, outlen + inlen);
>  	if (!retval)
> -		return NULL;
> +		return ERR_PTR(-ENOMEM);
>  
>  	if (outlen) {
>  		outreq.param_hdr.type = SCTP_PARAM_RESET_OUT_REQUEST;
> diff --git a/net/sctp/stream.c b/net/sctp/stream.c
> index fa8371f..51a25bf 100644
> --- a/net/sctp/stream.c
> +++ b/net/sctp/stream.c
> @@ -162,8 +162,8 @@ int sctp_send_reset_streams(struct sctp_association *asoc,
>  
>  	kfree(nstr_list);
>  
> -	if (!chunk) {
> -		retval = -ENOMEM;
> +	if (IS_ERR(chunk)) {
> +		retval = PTR_ERR(chunk);
>  		goto out;
>  	}
>  
> @@ -482,8 +482,10 @@ struct sctp_chunk *sctp_process_strreset_inreq(
>  	}
>  
>  	chunk = sctp_make_strreset_req(asoc, nums, str_p, 1, 0);
> -	if (!chunk)
> +	if (IS_ERR(chunk)) {
> +		chunk = NULL;
>  		goto out;
> +	}
>  
>  	if (nums)
>  		for (i = 0; i < nums; i++)
> -- 
> 2.1.0
> 
>