netdev - Re: [PATCH net] sctp: make sure stream nums can match optlen in sctp_setsockopt_reset

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Date:   Mon, 11 Dec 2017 09:54:34 -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>,
        syzkaller@...glegroups.com
Subject: Re: [PATCH net] sctp: make sure stream nums can match optlen in
 sctp_setsockopt_reset_streams

On Sun, Dec 10, 2017 at 03:40:51PM +0800, Xin Long wrote:
> Now in sctp_setsockopt_reset_streams, it only does the check
> optlen < sizeof(*params) for optlen. But it's not enough, as
> params->srs_number_streams should also match optlen.
> 
> If the streams in params->srs_stream_list are less than stream
> nums in params->srs_number_streams, later when dereferencing
> the stream list, it could cause a slab-out-of-bounds crash, as
> reported by syzbot.
> 
> This patch is to fix it by also checking the stream numbers in
> sctp_setsockopt_reset_streams to make sure at least it's not
> greater than the streams in the list.
> 
> Fixes: 7f9d68ac944e ("sctp: implement sender-side procedures for SSN Reset Request Parameter")
> Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
> Signed-off-by: Xin Long <lucien.xin@...il.com>
> ---
>  net/sctp/socket.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index 014847e..dbf140d 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -3891,13 +3891,17 @@ static int sctp_setsockopt_reset_streams(struct sock *sk,
>  	struct sctp_association *asoc;
>  	int retval = -EINVAL;
>  
> -	if (optlen < sizeof(struct sctp_reset_streams))
> +	if (optlen < sizeof(*params))
>  		return -EINVAL;
>  
Is this going to work in all corner cases?  IIRC struct sctp_reset_stream has
variable length array at the end of it, and so sizeof(struct sctp_reset_streams)
returns just the size of the struct, while sizeof(*params) returns the size of
the entire object (including the array elements).  If a user space task
allocates a static memory block to hold this struct and the array, and passes it
in with a shorter optlen (if for example, they do not intend to use all the
array elements), this will cause a failure, where no failure is truly warranted.
It seems the correct check to me should be the origional sizeof(struct
sctp_reset_streams) check, and the below check to ensure that there are at least
the same number of array elements available as indicated in srs_nuber_streams.

Regards
Neil

>  	params = memdup_user(optval, optlen);
>  	if (IS_ERR(params))
>  		return PTR_ERR(params);
>  
> +	if (params->srs_number_streams * sizeof(__u16) >
> +	    optlen - sizeof(*params))
> +		goto out;
> +
>  	asoc = sctp_id2assoc(sk, params->srs_assoc_id);
>  	if (!asoc)
>  		goto out;
> -- 
> 2.1.0
> 
> --
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