netdev - Re: [PATCH bpf-next] bpf/verifier: enable ctx + const + 0.

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Message-ID: <e4f93f62-d7f4-0851-d1c2-34a13c2af4c6@iogearbox.net>
Date:   Wed, 2 May 2018 10:29:32 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        William Tu <u9012063@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Yonghong Song <yhs@...com>, Yifeng Sun <pkusunyifeng@...il.com>
Subject: Re: [PATCH bpf-next] bpf/verifier: enable ctx + const + 0.

On 05/02/2018 06:52 AM, Alexei Starovoitov wrote:
> On Tue, May 01, 2018 at 09:35:29PM -0700, William Tu wrote:
>>
>>> How did you test this patch?
>>>
>> Without the patch, the test case will fail.
>> With the patch, the test case passes.
> 
> Please test it with real program and you'll see crashes and garbage returned.

+1, *convert_ctx_access() use bpf_insn's off to determine what to rewrite,
so this is definitely buggy, and wasn't properly tested as it should have
been. The test case is also way too simple, just the LDX and then doing a
return 0 will get you past verifier, but won't give you anything in terms
of runtime testing that test_verifier is doing. A single test case for a
non trivial verifier change like this is also _completely insufficient_,
this really needs to test all sort of weird corner cases (involving out of
bounds accesses, overflows, etc).