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Date:   Fri, 1 May 2020 15:52:55 -0700
From:   Alexei Starovoitov <>
To:     Martin KaFai Lau <>
Cc:     Andrii Nakryiko <>, bpf <>,
        Network Development <>,
        Alexei Starovoitov <>,
        Daniel Borkmann <>,
        Andrii Nakryiko <>,
        Kernel Team <>,
Subject: Re: [PATCH v2 bpf-next] bpf: fix use-after-free of bpf_link when
 priming half-fails

On Fri, May 1, 2020 at 1:00 PM Martin KaFai Lau <> wrote:
> On Fri, May 01, 2020 at 11:56:22AM -0700, Andrii Nakryiko wrote:
> > If bpf_link_prime() succeeds to allocate new anon file, but then fails to
> > allocate ID for it, link priming is considered to be failed and user is
> > supposed ot be able to directly kfree() bpf_link, because it was never exposed
> > to user-space.
> >
> > But at that point file already keeps a pointer to bpf_link and will eventually
> > call bpf_link_release(), so if bpf_link was kfree()'d by caller, that would
> > lead to use-after-free.
> >
> > Fix this by first allocating ID and only then allocating file. Adding ID to
> > link_idr is ok, because link at that point still doesn't have its ID set, so
> > no user-space process can create a new FD for it.
> Acked-by: Martin KaFai Lau <>

Applied. Thanks

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