Messages in this thread | | | From | "Reshetova, Elena" <> | Subject | RE: [RFC][PATCH 2/7] kref: Add kref_read() | Date | Fri, 18 Nov 2016 17:33:35 +0000 |
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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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