netdev - Re: sctp: bad hash index calculation

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Message-ID: <20160215155017.GB24321@hmsreliant.think-freely.org>
Date:	Mon, 15 Feb 2016 10:50:18 -0500
From:	Neil Horman <nhorman@...driver.com>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
	linux-sctp@...r.kernel.org,
	Vladislav Yasevich <vyasevich@...il.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>
Subject: Re: sctp: bad hash index calculation

On Mon, Feb 15, 2016 at 04:11:22PM +0100, Dmitry Vyukov wrote:
> Hello,
> 
> While looking into some memory leaks of sctp ports I've noticed that
> sctp_init initializes port hash table as follows:
> 
>     /* Allocate and initialize the SCTP port hash table.  */
>     do {
>         sctp_port_hashsize = (1UL << order) * PAGE_SIZE /
>                     sizeof(struct sctp_bind_hashbucket);
>         if ((sctp_port_hashsize > (64 * 1024)) && order > 0)
>             continue;
>         sctp_port_hashtable = (struct sctp_bind_hashbucket *)
>             __get_free_pages(GFP_KERNEL | __GFP_NOWARN, order);
>     } while (!sctp_port_hashtable && --order > 0);
> 
> and then hash index is computed as follows:
> 
> /* Warning: The following hash functions assume a power of two 'size'. */
> /* This is the hash function for the SCTP port hash table. */
> static inline int sctp_phashfn(struct net *net, __u16 lport)
> {
>     return (net_hash_mix(net) + lport) & (sctp_port_hashsize - 1);
> }
> 
> I don't see what ensures that sctp_port_hashsize is in fact a power-of-2.
> 
> spinlock_t in sctp_bind_hashbucket can be 2 words in some configs,
> then sizeof(sctp_bind_hashbucket) == 24, which can render half of hash
> table unused.
> 
> struct sctp_bind_hashbucket {
>         spinlock_t      lock;
>         struct hlist_head       chain;
> };
> 
> Am I missing something?
> 
You're right, its not.  It seems to me that sctp_port_hashsize is meant to
simply bound the upper index of the hashtable array, and as such the phashfn
should not assume that its a power of 2 (i.e. it should simply mod the hash
value by sctp_port_hashsize rather than and-ing it).  Alternatively we could
simply use alloc_large_system_hash to allocate this hash table here, the way tcp
does.  I'm traveling right now, but can take care of this as soon as i get home
on wednesday

Neil