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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Wed, 13 Apr 2022 12:49:58 -0700
Message-ID: <CAEf4BzbiVeQfhxEu908w2mU4d8+5kKeMknuvhzCXuxM9pJ1jmQ@mail.gmail.com>
Subject: Re: [PATCH] bpf: Fix KASAN use-after-free Read in compute_effective_progs
To:     Tadeusz Struk <tadeusz.struk@linaro.org>
Cc:     bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Networking <netdev@vger.kernel.org>,
        linux- stable <stable@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        syzbot+f264bffdfbd5614f3bb2@syzkaller.appspotmail.com
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On Wed, Apr 13, 2022 at 12:27 PM Tadeusz Struk <tadeusz.struk@linaro.org> wrote:
>
> On 4/13/22 12:07, Andrii Nakryiko wrote:
> >> it would be ideal if detach would never fail, but it would require some kind of
> >> prealloc, on attach maybe? Another option would be to minimize the probability
> > We allocate new arrays in update_effective_progs() under assumption
> > that we might need to grow the array because we use
> > update_effective_progs() for attachment. But for detachment we know
> > that we definitely don't need to increase the size, we need to remove
> > existing element only, thus shrinking the size.
> >
> > Normally we'd reallocate the array to shrink it (and that's why we use
> > update_effective_progs() and allocate memory), but we can also have a
> > fallback path for detachment only to reuse existing effective arrays
> > and just shift all the elements to the right from the element that's
> > being removed. We'll leave NULL at the end, but that's much better
> > than error out. Subsequent attachment or detachment will attempt to
> > properly size and reallocate everything.
> >
> > So I think that should be the fix, if you'd be willing to work on it.
>
> That makes it much easier then. I will change it so that there is no
> alloc needed on the detach path. Thanks for the clarification.

Keep in mind that we probably want to do normal alloc-based detach
first anyways, if it works. It will keep effective arrays minimally
sized. This additional detach specific logic should be a fall back
path if the normal way doesn't work.

>
> --
> Thanks,
> Tadeusz