netdev - Re: [PATCH bpf] bpf: fix redirect to map under tail calls

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Date:   Fri, 17 Aug 2018 15:59:35 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     john.fastabend@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH bpf] bpf: fix redirect to map under tail calls

On Fri, Aug 17, 2018 at 11:26:14PM +0200, Daniel Borkmann wrote:
> Commits 109980b894e9 ("bpf: don't select potentially stale ri->map
> from buggy xdp progs") and 7c3001313396 ("bpf: fix ri->map_owner
> pointer on bpf_prog_realloc") tried to mitigate that buggy programs
> using bpf_redirect_map() helper call do not leave stale maps behind.
> Idea was to add a map_owner cookie into the per CPU struct redirect_info
> which was set to prog->aux by the prog making the helper call as a
> proof that the map is not stale since the prog is implicitly holding
> a reference to it. This owner cookie could later on get compared with
> the program calling into BPF whether they match and therefore the
> redirect could proceed with processing the map safely.
> 
> In (obvious) hindsight, this approach breaks down when tail calls are
> involved since the original caller's prog->aux pointer does not have
> to match the one from one of the progs out of the tail call chain,
> and therefore the xdp buffer will be dropped instead of redirected.
> A way around that would be to fix the issue differently (which also
> allows to remove related work in fast path at the same time): once
> the life-time of a redirect map has come to its end we use it's map
> free callback where we need to wait on synchronize_rcu() for current
> outstanding xdp buffers and remove such a map pointer from the
> redirect info if found to be present. At that time no program is
> using this map anymore so we simply invalidate the map pointers to
> NULL iff they previously pointed to that instance while making sure
> that the redirect path only reads out the map once.
> 
> Fixes: 97f91a7cf04f ("bpf: add bpf_redirect_map helper routine")
> Fixes: 109980b894e9 ("bpf: don't select potentially stale ri->map from buggy xdp progs")
> Reported-by: Sebastiano Miano <sebastiano.miano@...ito.it>
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> Acked-by: John Fastabend <john.fastabend@...il.com>

Applied, Thanks