Messages in this thread | | | Subject | Re: [PATCH bpf-next v3 5/6] bpf: sockmap: allow update from BPF | From | Yonghong Song <> | Date | Fri, 21 Aug 2020 08:47:54 -0700 |
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On 8/21/20 3:29 AM, Lorenz Bauer wrote: > Allow calling bpf_map_update_elem on sockmap and sockhash from a BPF > context. The synchronization required for this is a bit fiddly: we > need to prevent the socket from changing its state while we add it > to the sockmap, since we rely on getting a callback via > sk_prot->unhash. However, we can't just lock_sock like in > sock_map_sk_acquire because that might sleep. So instead we disable > softirq processing and use bh_lock_sock to prevent further > modification. > > Yet, this is still not enough. BPF can be called in contexts where > the current CPU might have locked a socket. If the BPF can get > a hold of such a socket, inserting it into a sockmap would lead to > a deadlock. One straight forward example are sock_ops programs that > have ctx->sk, but the same problem exists for kprobes, etc. > We deal with this by allowing sockmap updates only from known safe > contexts. Improper usage is rejected by the verifier. > > I've audited the enabled contexts to make sure they can't run in > a locked context. It's possible that CGROUP_SKB and others are > safe as well, but the auditing here is much more difficult. In > any case, we can extend the safe contexts when the need arises. > > Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Acked-by: Yonghong Song <yhs@fb.com>
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