Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH v5 05/27] arm64: Use daifflag_restore after bp_hardening
To:     Julien Thierry <julien.thierry@arm.com>
Cc:     linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        daniel.thompson@linaro.org, joel@joelfernandes.org,
        marc.zyngier@arm.com, mark.rutland@arm.com,
        christoffer.dall@arm.com, catalin.marinas@arm.com,
        will.deacon@arm.com
References: <1535471497-38854-1-git-send-email-julien.thierry@arm.com>
 <1535471497-38854-6-git-send-email-julien.thierry@arm.com>
 <641119b9-3466-817d-3d6b-dd710c646b49@arm.com>
 <8f200315-1db9-1905-71ae-cb4269450b7c@arm.com>
From:   James Morse <james.morse@arm.com>
Message-ID: <71bb5289-b381-65da-c609-fa4a08640425@arm.com>
Date:   Wed, 12 Sep 2018 13:28:19 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.9.1
MIME-Version: 1.0
In-Reply-To: <8f200315-1db9-1905-71ae-cb4269450b7c@arm.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Hi Julien,

On 12/09/18 12:11, Julien Thierry wrote:
> On 12/09/18 11:32, James Morse wrote:
>> On 28/08/18 16:51, Julien Thierry wrote:
>>> For EL0 entries requiring bp_hardening, daif status is kept at
>>> DAIF_PROCCTX_NOIRQ until after hardening has been done. Then interrupts
>>> are enabled through local_irq_enable().
>>>
>>> Before using local_irq_* functions, daifflags should be properly restored
>>> to a state where IRQs are enabled.
>>
>>> Enable IRQs by restoring DAIF_PROCCTX state after bp hardening.
>>
>> Is this just for symmetry, or are you going on to add something to the daifflags
>> state that local_irq_* functions won't change? (if so, could you allude to that
>> in the commit message)

> What happens is that once we use ICC_PMR_EL1, local_irq_enable will not touch
> PSR.I. And we are coming back from an entry where PSR.I was kept to 1 so
> local_irq_enable was not actually enabling the interrupts. On the otherhand,
> restore will affect both.

Got it. Thanks!

Does this mean stop_machine()s local_save_flags()/local_irq_restore() will not
be symmetric around __apply_alternatives_multi_stop()?
I see you add alternatives in these in patch 15, but I haven't got that far yet)


> Another option is to have the asm macro "enable_da_f" also switch to PMR usage
> (i.e. "just keep normal interrupts disabled"). Overall it would probably be
> easier to reason with, but I'm just unsure whether it is acceptable to receive a
> Pseudo NMI before having applied the bp_hardening.

Wouldn't this give the interrupt controller a headache? I assume IRQs really are
masked when handle_arch_irq is called. (I know nothing about the gic)


Thanks,

James