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

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Date:   Mon, 22 Oct 2018 15:40:35 +0000
From:   Martin Lau <kafai@...com>
To:     Wenwen Wang <wang6495@....edu>
CC:     Kangjie Lu <kjlu@....edu>, Alexei Starovoitov <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        "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] bpf: btf: Fix a missing-check bug

On Fri, Oct 19, 2018 at 05:29:51PM -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.
> 
> To avoid the above issue, this patch rewrites the header after the second
> copy, using 'btf->hdr', which is obtained in the first copy.
Acked-by: Martin KaFai Lau <kafai@...com>