[net] sctp: set frag_point in sctp_setsockopt_maxseg correctly

Now in sctp_setsockopt_maxseg user_frag or frag_point can be set with
val >= 8 and val <= SCTP_MAX_CHUNK_LEN. But both checks are incorrect.

val >= 8 means frag_point can even be less than SCTP_DEFAULT_MINSEGMENT.
Then in sctp_datamsg_from_user(), when it's value is greater than cookie
echo len and trying to bundle with cookie echo chunk, the first_len will
overflow.

The worse case is when it's value is equal as cookie echo len, first_len
becomes 0, it will go into a dead loop for fragment later on. In Hangbin
syzkaller testing env, oom was even triggered due to consecutive memory
allocation in that loop.

Besides, SCTP_MAX_CHUNK_LEN is the max size of the whole chunk, it should
deduct the data header for frag_point or user_frag check.

This patch does a proper check with SCTP_DEFAULT_MINSEGMENT subtracting
the sctphdr and datahdr, SCTP_MAX_CHUNK_LEN subtracting datahdr when
setting frag_point via sockopt. It also improves sctp_setsockopt_maxseg
codes.

Suggested-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Reported-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
---
 include/net/sctp/sctp.h |  3 ++-
 net/sctp/socket.c       | 29 +++++++++++++++++++----------
 2 files changed, 21 insertions(+), 11 deletions(-)

On Fri, Nov 17, 2017 at 02:11:11PM +0800, Xin Long wrote:
> Now in sctp_setsockopt_maxseg user_frag or frag_point can be set with
> val >= 8 and val <= SCTP_MAX_CHUNK_LEN. But both checks are incorrect.
> 
> val >= 8 means frag_point can even be less than SCTP_DEFAULT_MINSEGMENT.
> Then in sctp_datamsg_from_user(), when it's value is greater than cookie
> echo len and trying to bundle with cookie echo chunk, the first_len will
> overflow.
> 
> The worse case is when it's value is equal as cookie echo len, first_len
> becomes 0, it will go into a dead loop for fragment later on. In Hangbin
> syzkaller testing env, oom was even triggered due to consecutive memory
> allocation in that loop.
> 
> Besides, SCTP_MAX_CHUNK_LEN is the max size of the whole chunk, it should
> deduct the data header for frag_point or user_frag check.
> 
> This patch does a proper check with SCTP_DEFAULT_MINSEGMENT subtracting
> the sctphdr and datahdr, SCTP_MAX_CHUNK_LEN subtracting datahdr when
> setting frag_point via sockopt. It also improves sctp_setsockopt_maxseg
> codes.
> 
> Suggested-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
> Reported-by: Hangbin Liu <liuhangbin@gmail.com>
> Signed-off-by: Xin Long <lucien.xin@gmail.com>

Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>

> ---
>  include/net/sctp/sctp.h |  3 ++-
>  net/sctp/socket.c       | 29 +++++++++++++++++++----------
>  2 files changed, 21 insertions(+), 11 deletions(-)
> 
> diff --git a/include/net/sctp/sctp.h b/include/net/sctp/sctp.h
> index d7d8cba..749a428 100644
> --- a/include/net/sctp/sctp.h
> +++ b/include/net/sctp/sctp.h
> @@ -444,7 +444,8 @@ static inline int sctp_frag_point(const struct sctp_association *asoc, int pmtu)
>  	if (asoc->user_frag)
>  		frag = min_t(int, frag, asoc->user_frag);
>  
> -	frag = SCTP_TRUNC4(min_t(int, frag, SCTP_MAX_CHUNK_LEN));
> +	frag = SCTP_TRUNC4(min_t(int, frag, SCTP_MAX_CHUNK_LEN -
> +					    sizeof(struct sctp_data_chunk)));
>  
>  	return frag;
>  }
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index 4c0a772..3204a9b 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -3140,9 +3140,9 @@ static int sctp_setsockopt_mappedv4(struct sock *sk, char __user *optval, unsign
>   */
>  static int sctp_setsockopt_maxseg(struct sock *sk, char __user *optval, unsigned int optlen)
>  {
> +	struct sctp_sock *sp = sctp_sk(sk);
>  	struct sctp_assoc_value params;
>  	struct sctp_association *asoc;
> -	struct sctp_sock *sp = sctp_sk(sk);
>  	int val;
>  
>  	if (optlen == sizeof(int)) {
> @@ -3158,26 +3158,35 @@ static int sctp_setsockopt_maxseg(struct sock *sk, char __user *optval, unsigned
>  		if (copy_from_user(&params, optval, optlen))
>  			return -EFAULT;
>  		val = params.assoc_value;
> -	} else
> +	} else {
>  		return -EINVAL;
> +	}
>  
> -	if ((val != 0) && ((val < 8) || (val > SCTP_MAX_CHUNK_LEN)))
> -		return -EINVAL;
> +	if (val) {
> +		int min_len, max_len;
>  
> -	asoc = sctp_id2assoc(sk, params.assoc_id);
> -	if (!asoc && params.assoc_id && sctp_style(sk, UDP))
> -		return -EINVAL;
> +		min_len = SCTP_DEFAULT_MINSEGMENT - sp->pf->af->net_header_len;
> +		min_len -= sizeof(struct sctphdr) +
> +			   sizeof(struct sctp_data_chunk);
> +
> +		max_len = SCTP_MAX_CHUNK_LEN - sizeof(struct sctp_data_chunk);
>  
> +		if (val < min_len || val > max_len)
> +			return -EINVAL;
> +	}
> +
> +	asoc = sctp_id2assoc(sk, params.assoc_id);
>  	if (asoc) {
>  		if (val == 0) {
> -			val = asoc->pathmtu;
> -			val -= sp->pf->af->net_header_len;
> +			val = asoc->pathmtu - sp->pf->af->net_header_len;
>  			val -= sizeof(struct sctphdr) +
> -					sizeof(struct sctp_data_chunk);
> +			       sizeof(struct sctp_data_chunk);
>  		}
>  		asoc->user_frag = val;
>  		asoc->frag_point = sctp_frag_point(asoc, asoc->pathmtu);
>  	} else {
> +		if (params.assoc_id && sctp_style(sk, UDP))
> +			return -EINVAL;
>  		sp->user_frag = val;
>  	}
>  
> -- 
> 2.1.0
>

From: Xin Long <lucien.xin@gmail.com>
Date: Fri, 17 Nov 2017 14:11:11 +0800

> Now in sctp_setsockopt_maxseg user_frag or frag_point can be set with
> val >= 8 and val <= SCTP_MAX_CHUNK_LEN. But both checks are incorrect.
> 
> val >= 8 means frag_point can even be less than SCTP_DEFAULT_MINSEGMENT.
> Then in sctp_datamsg_from_user(), when it's value is greater than cookie
> echo len and trying to bundle with cookie echo chunk, the first_len will
> overflow.
> 
> The worse case is when it's value is equal as cookie echo len, first_len
> becomes 0, it will go into a dead loop for fragment later on. In Hangbin
> syzkaller testing env, oom was even triggered due to consecutive memory
> allocation in that loop.
> 
> Besides, SCTP_MAX_CHUNK_LEN is the max size of the whole chunk, it should
> deduct the data header for frag_point or user_frag check.
> 
> This patch does a proper check with SCTP_DEFAULT_MINSEGMENT subtracting
> the sctphdr and datahdr, SCTP_MAX_CHUNK_LEN subtracting datahdr when
> setting frag_point via sockopt. It also improves sctp_setsockopt_maxseg
> codes.
> 
> Suggested-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
> Reported-by: Hangbin Liu <liuhangbin@gmail.com>
> Signed-off-by: Xin Long <lucien.xin@gmail.com>

Applied.