linux-kernel - Re: [PATCH v5 05/27] arm64: Use daifflag_restore after bp

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Date:   Wed, 12 Sep 2018 12:11:15 +0100
From:   Julien Thierry <julien.thierry@....com>
To:     James Morse <james.morse@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        daniel.thompson@...aro.org, joel@...lfernandes.org,
        marc.zyngier@....com, mark.rutland@....com,
        christoffer.dall@....com, catalin.marinas@....com,
        will.deacon@....com
Subject: Re: [PATCH v5 05/27] arm64: Use daifflag_restore after bp_hardening

Hi James,

On 12/09/18 11:32, James Morse wrote:
> Hi Julien,
> 
> 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.

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.

If it is possible, I'm happy to solve this with enable_da_f.

Thanks,

> 
> Either way,
> 
> Acked-by: James Morse <james.morse@....com>
> 

-- 
Julien Thierry