RE: [RFC][PATCH 2/7] kref: Add kref_read()

[Reshetova, Elena]

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@xxxxxxxxxxxxxxxxxxxxxxx
> -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...