netdev - Re: [PATCH] 9p: fix Use-After-Free in p9_write

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Date:   Mon, 30 Jul 2018 12:23:49 +0200
From:   Dominique Martinet <asmadeus@...ewreck.org>
To:     Tomas Bortoli <tomasbortoli@...il.com>
Cc:     davem@...emloft.net, v9fs-developer@...ts.sourceforge.net,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller@...glegroups.com
Subject: Re: [PATCH] 9p: fix Use-After-Free in p9_write_work()

Tomas Bortoli wrote on Mon, Jul 30, 2018:
> > Other transports also have the same issue see discussion in
> > https://lkml.org/lkml/2018/7/19/727
> > (that is another syzbot report, slightly different but I believe it
> > points to the same issue)
> > 
> > Basically, a more global view of the problem is a race between
> > p9_tag_lookup returning a p9_req_t and another thread freeing it.
> > 
> > Matthew wrote the problem himself in a comment in p9_tag_lookup in his new
> > version that used to be in linux-next at the time (I took the commit out
> > temporarily until I've had time to benchmark it, but it will come back in,
> > just you're working on thin air right now because the bug was only found
> > thanks to this commit):
> > +       /* There's no refcount on the req; a malicious server could
> > cause
> > +        * us to dereference a NULL pointer
> > +        */
> > 
> > So a more proper solution would be to had a refcount to req, have
> > p9_tag_lookup increment the refcount within rcu_read_lock, and have a
> > deref function free the req when the count hits 0.
> 
> Which commit ? that's a comment.

Sorry, the commit is this one:
http://lkml.kernel.org/r/20180711210225.19730-6-willy@infradead.org

It's now out of my 9p-next branch due to performance reasons but I'll
definitely take it back in once my performance mitigation patches have
had a few reviews.

> That sound like the proper solution. Let's do it that way then.

Cool :)

-- 
Dominique