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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Nov 15, 2016 at 12:53:35PM -0800, Kees Cook wrote:
> 
> What should we do about things like this (bpf_prog_put() and callbacks
> from kernel/bpf/syscall.c):
> 
> 
> static void bpf_prog_uncharge_memlock(struct bpf_prog *prog)
> {
>         struct user_struct *user = prog->aux->user;
> 
>         atomic_long_sub(prog->pages, &user->locked_vm);
>         free_uid(user);
> }
> 
> static void __bpf_prog_put_rcu(struct rcu_head *rcu)
> {
>         struct bpf_prog_aux *aux = container_of(rcu, struct bpf_prog_aux, rcu);
> 
>         free_used_maps(aux);
>         bpf_prog_uncharge_memlock(aux->prog);
>         bpf_prog_free(aux->prog);
> }
> 
> void bpf_prog_put(struct bpf_prog *prog)
> {
>         if (atomic_dec_and_test(&prog->aux->refcnt))
>                 call_rcu(&prog->aux->rcu, __bpf_prog_put_rcu);
> }
> 
> 
> Not only do we want to protect prog->aux->refcnt, but I think we want
> to protect user->locked_vm too ... I don't think it's sane for
> user->locked_vm to be a stats_t ?

Why would you want to mess with locked_vm? You seem of the opinion that
everything atomic_t is broken, this isn't the case.