linux-kernel - Re: [PATCH v2] ARM: spectre-v2: fix smp_processor

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Message-ID: <6d84da13-8c7b-129e-a40c-cadc80a609f4@I-love.SAKURA.ne.jp>
Date:   Fri, 15 Jul 2022 23:07:27 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Marc Zyngier <maz@...nel.org>, Tony Lindgren <tony@...mide.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v2] ARM: spectre-v2: fix smp_processor_id() warning

On 2022/07/15 22:36, Russell King (Oracle) wrote:
> On Fri, Jul 15, 2022 at 10:09:01PM +0900, Tetsuo Handa wrote:
>> syzbot is reporting that CONFIG_HARDEN_BRANCH_PREDICTOR=y +
>> CONFIG_DEBUG_PREEMPT=y on ARM32 causes "BUG: using smp_processor_id() in
>> preemptible code" message [1], for this check was not designed to handle
>> attempts to access kernel memory like
>>
>>   ----------
>>   int main() { return *(char *) -1; }
>>   ----------
>>
>> . Although Russell King commented that this BUG: message might help finding
>> possible exploit attempts [2], this is not a kernel's problem that worth
>> giving up fuzz testing.
> 
> But shutting up a valid warning when the real problem is still there is
> also not acceptable.

Then, at least for now can we stop emitting the BUG: string? I showed an idea at
https://lkml.kernel.org/r/fa786d1c-db06-f7f1-9ac9-6a468c1e8d81@I-love.SAKURA.ne.jp
but I got no response.

Since syzkaller stops fuzz testing upon encountering BUG: or WARNING: string,
ARM32 might be failing to find other bugs for 491 days due to this problem.

If you don't want to stop emitting this BUG: string, we might want to teach
syzkaller to build ARM32 kernels with CONFIG_HARDEN_BRANCH_PREDICTOR=n.