netdev - Re: [PATCH net] ptr_ring: use kmalloc

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Date:   Fri, 25 Aug 2017 11:57:19 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>, Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH net] ptr_ring: use kmalloc_array()

On Fri, 2017-08-25 at 21:03 +0300, Michael S. Tsirkin wrote:
> On Wed, Aug 16, 2017 at 10:36:47AM -0700, Eric Dumazet wrote:
> > From: Eric Dumazet <edumazet@...gle.com>
> > 
> > As found by syzkaller, malicious users can set whatever tx_queue_len
> > on a tun device and eventually crash the kernel.
> > 
> > Lets remove the ALIGN(XXX, SMP_CACHE_BYTES) thing since a small
> > ring buffer is not fast anyway.
> 
> I'm not sure it's worth changing for small rings.
> 
> Does kmalloc_array guarantee cache line alignment for big buffers
> then? If the ring is misaligned it will likely cause false sharing
> as it's designed to be accessed from two CPUs.

I specifically said that in the changelog :

"since a small ring buffer is not fast anyway."

If one user sets up a pathological small ring buffer, kernel should not
try to fix it.

In this case, you would have to setup a ring of 2 or 4 slots to
eventually hit false sharing.