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Subject: Re: [PATCH bpf v5 0/2] Fix bpf_probe_read_user_str() overcopying
From:   "Daniel Xu" <dxu@dxuuu.xyz>
To:     "Andrii Nakryiko" <andrii.nakryiko@gmail.com>
Cc:     "bpf" <bpf@vger.kernel.org>,
        "open list" <linux-kernel@vger.kernel.org>,
        "Alexei Starovoitov" <ast@kernel.org>,
        "Daniel Borkmann" <daniel@iogearbox.net>,
        "Song Liu" <songliubraving@fb.com>,
        "Kernel Team" <kernel-team@fb.com>
Date:   Thu, 12 Nov 2020 11:10:59 -0800
Message-Id: <C71IU5Z0R6UI.29FQP3BCZ65ZC@maharaja>
In-Reply-To: <CAEf4BzZx=7N6dbKk8Eb_k-FA-PmmPFBJ=V-PLhbDu38wuXkOkw@mail.gmail.com>
Precedence: bulk

On Wed Nov 11, 2020 at 3:22 PM PST, Andrii Nakryiko wrote:
> On Wed, Nov 11, 2020 at 2:46 PM Daniel Xu <dxu@dxuuu.xyz> wrote:
> >
> > 6ae08ae3dea2 ("bpf: Add probe_read_{user, kernel} and probe_read_{user,
> > kernel}_str helpers") introduced a subtle bug where
> > bpf_probe_read_user_str() would potentially copy a few extra bytes afte=
r
> > the NUL terminator.
> >
> > This issue is particularly nefarious when strings are used as map keys,
> > as seemingly identical strings can occupy multiple entries in a map.
> >
> > This patchset fixes the issue and introduces a selftest to prevent
> > future regressions.
> >
> > v4 -> v5:
> > * don't read potentially uninitialized memory
>
> I think the bigger problem was that it could overwrite unintended
> memory. E.g., in BPF program, if you had something like:
>
> char my_buf[8 + 3];
> char my_precious_data[5] =3D {1, 2, 3, 4, 5};

How does that happen?

The

    while (max >=3D sizeof(unsigned long)) {
            /* copy 4 bytes */

            max -=3D sizeof(unsigned long)
    }

    /* copy byte at a time */

where `max` is the user supplied length should prevent that kind of
corruption, right?

[...]