linux-kernel - Re: [PATCH v2] bpf: btf: Fix a missing-check bug

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Message-ID: <7a5a0bb9-ce61-b9e8-7362-32dac8823700@iogearbox.net>
Date:   Fri, 26 Oct 2018 00:58:31 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Martin Lau <kafai@...com>, Wenwen Wang <wang6495@....edu>
Cc:     Kangjie Lu <kjlu@....edu>, Alexei Starovoitov <ast@...nel.org>,
        "open list:BPF (Safe dynamic programs and tools)" 
        <netdev@...r.kernel.org>,
        "open list:BPF (Safe dynamic programs and tools)" 
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] bpf: btf: Fix a missing-check bug

On 10/24/2018 10:42 PM, Martin Lau wrote:
> On Wed, Oct 24, 2018 at 06:22:46PM +0000, Martin Lau wrote:
>> On Wed, Oct 24, 2018 at 05:26:23PM +0000, Martin Lau wrote:
>>> On Wed, Oct 24, 2018 at 08:00:19AM -0500, Wenwen Wang wrote:
>>>> In btf_parse(), the header of the user-space btf data 'btf_data' is firstly
>>>> parsed and verified through btf_parse_hdr(). In btf_parse_hdr(), the header
>>>> is copied from user-space 'btf_data' to kernel-space 'btf->hdr' and then
>>>> verified. If no error happens during the verification process, the whole
>>>> data of 'btf_data', including the header, is then copied to 'data' in
>>>> btf_parse(). It is obvious that the header is copied twice here. More
>>>> importantly, no check is enforced after the second copy to make sure the
>>>> headers obtained in these two copies are same. Given that 'btf_data'
>>>> resides in the user space, a malicious user can race to modify the header
>>>> between these two copies. By doing so, the user can inject inconsistent
>>>> data, which can cause undefined behavior of the kernel and introduce
>>>> potential security risk.
>> btw, I am working on a patch that copies the btf_data before parsing/verifying
>> the header.  That should avoid this from happening but that will
>> require a bit more code churns for the bpf branch.
>>
> It is what I have in mind:
> 
> It is not a good idea to check the BTF header before copying the
> user btf_data.  The verified header may not be the one actually
> copied to btf->data (e.g. userspace may modify the passed in
> btf_data in between).  Like the one fixed in
> commit 8af03d1ae2e1 ("bpf: btf: Fix a missing check bug").
> 
> This patch copies the user btf_data before parsing/verifying
> the BTF header.
> 
> Fixes: 69b693f0aefa ("bpf: btf: Introduce BPF Type Format (BTF)")
> Signed-off-by: Martin KaFai Lau <kafai@...com>

I've added Co-developed-by tag and applied it to bpf tree, thanks everyone!