netdev - Re: [PATCH v2 bpf-next] bpf: fix use-after-free of bpf

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Message-ID: <20200501195929.4s6y7wee3u5usk4n@kafai-mbp>
Date:   Fri, 1 May 2020 12:59:29 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Andrii Nakryiko <andriin@...com>
CC:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>, <ast@...com>,
        <daniel@...earbox.net>, <andrii.nakryiko@...il.com>,
        <kernel-team@...com>,
        <syzbot+39b64425f91b5aab714d@...kaller.appspotmail.com>
Subject: Re: [PATCH v2 bpf-next] bpf: fix use-after-free of bpf_link when
 priming half-fails

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 <kafai@...com>