linux-kernel - RE: [RFC][PATCH 2/7] kref: Add kref

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Date:   Mon, 21 Nov 2016 08:18:04 +0000
From:   "Reshetova, Elena" <elena.reshetova@...el.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        David Windsor <dave@...gbits.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: RE: [RFC][PATCH 2/7] kref: Add kref_read()

> On Fri, Nov 18, 2016 at 05:33:35PM +0000, Reshetova, Elena wrote:
> > On Thu, Nov 17, 2016 at 09:53:42AM +0100, Peter Zijlstra wrote:
> > > On Wed, Nov 16, 2016 at 12:08:52PM -0800, Alexei Starovoitov wrote:
> > >
> > > > I prefer to avoid 'fixing' things that are not broken.
> > > > Note, prog->aux->refcnt already has explicit checks for overflow.
> > > > locked_vm is used for resource accounting and not refcnt, so I don't
> > > > see issues there either.
> > >
> > > The idea is to use something along the lines of:
> > >
> > >
> > >
> http://lkml.kernel.org/r/20161115104608.GH3142@twins.programming.kicks
> > > -ass.net
> > >
> > > for all refcounts in the kernel.
> >
> > >I understand the idea. I'm advocating to fix refcnts explicitly the way we did
> in bpf land instead of leaking memory, making processes unkillable and so on.
> > >If refcnt can be bounds checked, it should be done that way, since it's a
> clean error path without odd side effects.
> > >Therefore I'm against unconditionally applying refcount to all atomics.
> >
> > > Also note that your:
> > >
> > > struct bpf_prog *bpf_prog_add(struct bpf_prog *prog, int i) {
> > >         if (atomic_add_return(i, &prog->aux->refcnt) > BPF_MAX_REFCNT) {
> > >                 atomic_sub(i, &prog->aux->refcnt);
> > >                 return ERR_PTR(-EBUSY);
> > >         }
> > >         return prog;
> > > }
> > >
> > > is actually broken in the face of an actual overflow. Suppose @i is
> > > big enough to wrap refcnt into negative space.
> >
> > >'i' is not controlled by user. It's a number of nic hw queues and
> BPF_MAX_REFCNT is 32k, so above is always safe.
> >
> > If I understand your code right, you export the bpf_prog_add() and anyone is
> free to use it
> > (some crazy buggy driver for example).
> > Currently only drivers/net/ethernet/mellanox/mlx4/en_netdev.c uses it, but
> you should
> > consider any externally exposed interface as an attack vector from security
> point of view.
> 
> It's not realistic to harden all export_symbol apis.
> Code review for in-tree modules is the only defense we have.
> Remember out of tree perf counter issues... nothing perf core can do
> about that. If it's out of tree, it's vendor's problem to fix it.

I am not trying to harden them all now, but since we are going through the list of 
atomic_t variables that are used for refcounting, and this seems to be the one, 
I was trying to find a way to convert it also since it isn't a big effort and would do 
good at the end. 
So, you are not fine if I convert the above code using only refcount_inc and refcount_dec_and_test 
primitives?