linux-kernel - Re: [PATCH net] vhost: Use kzalloc() to allocate vhost_msg

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Message-ID: <0dcd15ae-cd9b-1e3c-1311-4d86d1aa51d2@redhat.com>
Date:   Sat, 28 Apr 2018 10:23:18 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Kevin Easton <kevin@...rana.org>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH net] vhost: Use kzalloc() to allocate vhost_msg_node



On 2018年04月28日 09:51, Kevin Easton wrote:
> On Fri, Apr 27, 2018 at 09:07:56PM -0400, Kevin Easton wrote:
>> On Fri, Apr 27, 2018 at 07:05:45PM +0300, Michael S. Tsirkin wrote:
>>> On Fri, Apr 27, 2018 at 11:45:02AM -0400, Kevin Easton wrote:
>>>> The struct vhost_msg within struct vhost_msg_node is copied to userspace,
>>>> so it should be allocated with kzalloc() to ensure all structure padding
>>>> is zeroed.
>>>>
>>>> Signed-off-by: Kevin Easton <kevin@...rana.org>
>>>> Reported-by: syzbot+87cfa083e727a224754b@...kaller.appspotmail.com
>>> Does it help if a patch naming the padding is applied,
>>> and then we init just the relevant field?
>>> Just curious.
>> No, I don't believe that is sufficient to fix the problem.
> Scratch that, somehow I missed the "..and then we init just the
> relevant field" part, sorry.
>
> There's still the padding after the vhost_iotlb_msg to consider.  It's
> named in the union but I don't think that's guaranteed to be initialised
> when the iotlb member of the union is used to initialise things.
>
>> I didn't name the padding in my original patch because I wasn't sure
>> if the padding actually exists on 32 bit architectures?
> This might still be a conce

Yes.

print &((struct vhost_msg *)0)->iotlb
$3 = (struct vhost_iotlb_msg *) 0x4


>
> At the end of the day, zeroing 96 bytes (the full size of vhost_msg_node)
> is pretty quick.
>
>      - Kevin

Right, and even if it may be used heavily in the data-path, zeroing is 
not the main delay in that path.

Thanks