Re: [PATCH net v2] sctp: fix a potential buffer overflow in sctp_sched_set_sched()

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From:   Gavrilov Ilia <Ilia.Gavrilov@infotecs.ru>
Date:   Tue, 2 May 2023 13:03:24 +0000
> The 'sched' index value must be checked before accessing an element
> of the 'sctp_sched_ops' array. Otherwise, it can lead to buffer overflow.

OOB access ?
But it's not true because it does not happen in the first place.

> 
> Note that it's harmless since the 'sched' parameter is checked before
> calling 'sctp_sched_set_sched'.
> 
> Found by InfoTeCS on behalf of Linux Verification Center
> (linuxtesting.org) with SVACE.
> 
> Fixes: 5bbbbe32a431 ("sctp: introduce stream scheduler foundations")
> Reviewed-by: Simon Horman <simon.horman@corigine.com>
> Signed-off-by: Ilia.Gavrilov <Ilia.Gavrilov@infotecs.ru>
> ---
> V2:
>  - Change the order of local variables 
>  - Specify the target tree in the subject
>  net/sctp/stream_sched.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/net/sctp/stream_sched.c b/net/sctp/stream_sched.c
> index 330067002deb..4d076a9b8592 100644
> --- a/net/sctp/stream_sched.c
> +++ b/net/sctp/stream_sched.c
> @@ -146,18 +146,19 @@ static void sctp_sched_free_sched(struct sctp_stream *stream)
>  int sctp_sched_set_sched(struct sctp_association *asoc,
>  			 enum sctp_sched_type sched)
>  {
> -	struct sctp_sched_ops *n = sctp_sched_ops[sched];
>  	struct sctp_sched_ops *old = asoc->outqueue.sched;
>  	struct sctp_datamsg *msg = NULL;
> +	struct sctp_sched_ops *n;
>  	struct sctp_chunk *ch;
>  	int i, ret = 0;
>  
> -	if (old == n)
> -		return ret;
> -
>  	if (sched > SCTP_SS_MAX)
>  		return -EINVAL;

I'd just remove this check instead because the same test is done
in the caller side, sctp_setsockopt_scheduler(), and this errno
is never returned.

This unnecessary test confuses a reader like sched could be over
SCTP_SS_MAX here.

Since the OOB access does not happen, I think this patch should
go to net-next without the Fixes tag after the merge window.

Thanks,
Kuniyuki


>  
> +	n = sctp_sched_ops[sched];
> +	if (old == n)
> +		return ret;
> +
>  	if (old)
>  		sctp_sched_free_sched(&asoc->stream);
>  
> -- 
> 2.30.2