Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <bb29d766-f837-195e-63cc-15d02f155f2c@linaro.org>
Date:   Wed, 13 Apr 2022 12:27:15 -0700
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.7.0
Subject: Re: [PATCH] bpf: Fix KASAN use-after-free Read in
 compute_effective_progs
Content-Language: en-US
To:     Andrii Nakryiko <andrii.nakryiko@gmail.com>
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
References: <20220405170356.43128-1-tadeusz.struk@linaro.org>
 <CAEf4BzaPmp5TzNM8U=SSyEp30wv335_ZxuAL-LLPQUZJ9OS74g@mail.gmail.com>
 <e7692d0b-e495-8d3e-4905-c4109bf5caa4@linaro.org>
 <CAEf4Bzbb+AmuABH2cw=48uuznz7bT=eEMc1V9mS3GSqgU664Tw@mail.gmail.com>
From:   Tadeusz Struk <tadeusz.struk@linaro.org>
In-Reply-To: <CAEf4Bzbb+AmuABH2cw=48uuznz7bT=eEMc1V9mS3GSqgU664Tw@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

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