linux-kernel - Re: [PATCH] vhost: zero vhost

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Date:   Thu, 28 Mar 2019 16:36:47 +0000
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Vitaly Mayatskih <v.mayatskih@...il.com>
Cc:     Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vhost: zero vhost_vsock memory on allocation

On Wed, Mar 27, 2019 at 01:08:53PM -0400, Vitaly Mayatskih wrote:
> On Wed, Mar 27, 2019 at 12:49 PM Stefan Hajnoczi <stefanha@...hat.com> wrote:
> 
> > Which field was accessed before initialization?
> >
> > I ask because the situation is now unclear since code remains that
> > assumes vsock is *not* zero-initialized:
> >
> >   vsock->guest_cid = 0; /* no CID assigned yet */
> >
> >   atomic_set(&vsock->queued_replies, 0);
> 
> It was hash.

vsock->hash is only read if vsock->guest_cid has already been set and
hence vsock->hash has been initialized too.  I don't see where the
problem is.

Was your tree missing commit a72b69dc083a931422cc8a5e33841aff7d5312f2
("vhost/vsock: fix uninitialized vhost_vsock->guest_cid")?

Are you sure the crash can be reproduced on linux.git/master?

Sorry for insisting on so much information but I want to make I fully
understand the issue you encountered.

> > If we're going to zalloc, let's get rid of explicit zero
> > initializations.  Or let's use kvmalloc() and fix the uninitialized
> > access.  Mixing both is confusing.
> 
> I would go with zalloc, since it's easier to read and it prevents
> further situations like this.
> A zalloc was there originally (not in fallback though).

Sounds good.  Please send a v2 that also removes the explicit zero
initialization since it's no longer needed with zalloc.

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