linux-kernel - Re: Allocation failure with subsequent kernel crash

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Message-ID: <CAP8WD_ahR75c7OutCSbPKdt43QUpBB+Eup-E_b9dHaoTqTq5cw@mail.gmail.com>
Date:   Wed, 5 Sep 2018 10:39:34 -0400
From:   tedheadster <tedheadster@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Matthew Whitehead <whiteheadm@....org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: Allocation failure with subsequent kernel crash

> I've been looking into it a bit today and still am. Given you've seen
> this on x86_32 and also on older kernels, I presume JIT was not involved
> (/proc/sys/net/core/bpf_jit_enable is 0). Do you run any specific workload
> until you trigger this (e.g. fuzzer on BPF), or any specific event that
> triggers at that time after ~5hrs? Or only systemd on idle machine? Have
> you managed to reproduce this also elsewhere? Bisect seems indeed painful
> but would help tremendously; perhaps also dumping the BPF insns that are
> loaded at that point in time.

Daniel,
  I've been trying for days to bisect this, but it is hard to
reproduce. However, I did have a question.

The crash is happening when bpf_prog_load() hits an error case and
then jumps to free_used_maps(prog->aux). However, I don't see an
obvious place where the 'aux' field gets initialized in
bpf_prog_load(). So it might easily be zero/null.

Could that explain the crash due to "unable to handle kernel NULL
pointer dereference"?

- Matthew