netdev - Re: [PATCH] bpf: Fix KASAN use-after-free Read in compute_effective

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Date:   Wed, 13 Apr 2022 12:27:15 -0700
From:   Tadeusz Struk <tadeusz.struk@...aro.org>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        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>,
        Networking <netdev@...r.kernel.org>,
        linux- stable <stable@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        syzbot+f264bffdfbd5614f3bb2@...kaller.appspotmail.com
Subject: Re: [PATCH] bpf: Fix KASAN use-after-free Read in
 compute_effective_progs

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.

-- 
Thanks,
Tadeusz