netdev - Re: [PATCH net-next] bpf: fix double free from dev_map

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Date:   Sun, 20 Aug 2017 17:42:45 -0700
From:   Alexei Starovoitov <ast@...com>
To:     Daniel Borkmann <daniel@...earbox.net>, <davem@...emloft.net>
CC:     <john.fastabend@...il.com>, <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] bpf: fix double free from dev_map_notification()

On 8/20/17 4:48 PM, Daniel Borkmann wrote:
> In the current code, dev_map_free() can still race with dev_map_notification().
> In dev_map_free(), we remove dtab from the list of dtabs after we purged
> all entries from it. However, we don't do xchg() with NULL or the like,
> so the entry at that point is still pointing to the device. If a unregister
> notification comes in at the same time, we therefore risk a double-free,
> since the pointer is still present in the map, and then pushed again to
> __dev_map_entry_free().
>
> All this is completely unnecessary. Just remove the dtab from the list
> right before the synchronize_rcu(), so all outstanding readers from the
> notifier list have finished by then, thus we don't need to deal with this
> corner case anymore and also wouldn't need to nullify dev entires. This is
> fine because we iterate over the map releasing all entries and therefore
> dev references anyway.
>
> Fixes: 4cc7b9544b9a ("bpf: devmap fix mutex in rcu critical section")
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>

makes sense to me
Acked-by: Alexei Starovoitov <ast@...nel.org>
I wonder why it was done the other way around in the first place then?
dev_map_list is there only for notifier and since the map is freed
with all the devices totally makes sense to isolate it from notifier
as a first step.