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Message-ID: <2236FBA76BA1254E88B949DDB74E612B41C14BB4@IRSMSX102.ger.corp.intel.com>
Date: Fri, 18 Nov 2016 17:33:35 +0000
From: "Reshetova, Elena" <elena.reshetova@...el.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>,
Peter Zijlstra <peterz@...radead.org>
CC: 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>,
Daniel Borkmann <daniel@...earbox.net>
Subject: RE: [RFC][PATCH 2/7] kref: Add kref_read()
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.
So, I would not claim that above construction is always safe since there is a way using API to
supply "i" that would overflow.
Next question is how to convert the above code sanely to refcount_t interface... Loop of inc(s)? Iikk...
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