netdev - Re: [PATCH bpf] bpf: Fix the off-by-two error in range markings

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Date:   Tue, 30 Nov 2021 22:40:17 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Maxim Mikityanskiy <maximmi@...dia.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>
Cc:     Shuah Khan <shuah@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        bpf@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH bpf] bpf: Fix the off-by-two error in range markings

On 11/30/21 7:16 PM, Maxim Mikityanskiy wrote:
> The first commit cited below attempts to fix the off-by-one error that
> appeared in some comparisons with an open range. Due to this error,
> arithmetically equivalent pieces of code could get different verdicts
> from the verifier, for example (pseudocode):
> 
>    // 1. Passes the verifier:
>    if (data + 8 > data_end)
>        return early
>    read *(u64 *)data, i.e. [data; data+7]
> 
>    // 2. Rejected by the verifier (should still pass):
>    if (data + 7 >= data_end)
>        return early
>    read *(u64 *)data, i.e. [data; data+7]
> 
> The attempted fix, however, shifts the range by one in a wrong
> direction, so the bug not only remains, but also such piece of code
> starts failing in the verifier:
> 
>    // 3. Rejected by the verifier, but the check is stricter than in #1.
>    if (data + 8 >= data_end)
>        return early
>    read *(u64 *)data, i.e. [data; data+7]
> 
> The change performed by that fix converted an off-by-one bug into
> off-by-two. The second commit cited below added the BPF selftests
> written to ensure than code chunks like #3 are rejected, however,
> they should be accepted.
> 
> This commit fixes the off-by-two error by adjusting new_range in the
> right direction and fixes the tests by changing the range into the one
> that should actually fail.
> 
> Fixes: fb2a311a31d3 ("bpf: fix off by one for range markings with L{T, E} patterns")
> Fixes: b37242c773b2 ("bpf: add test cases to bpf selftests to cover all access tests")
> Signed-off-by: Maxim Mikityanskiy <maximmi@...dia.com>
> ---
> After this patch is merged, I'm going to submit another patch to
> bpf-next, that will add new selftests for this bug.

Thanks for the fix, pls post the selftests for bpf tree; it's okay to route
them via bpf so they can go via CI for both trees eventually.