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Subject: Re: [PATCH] arm64/io: Don't use WZR in writel
To:     Will Deacon <will.deacon@arm.com>,
        AngeloGioacchino Del Regno <kholk11@gmail.com>
Cc:     Jens Axboe <axboe@kernel.dk>,
        Catalin Marinas <catalin.marinas@arm.com>,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-arm-msm@vger.kernel.org
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From:   Marc Zyngier <marc.zyngier@arm.com>
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Organization: ARM Ltd
Message-ID: <38d8965a-cd41-17cf-1b95-8dd58c079be4@arm.com>
Date:   Mon, 11 Feb 2019 11:52:52 +0000
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On 11/02/2019 10:57, Will Deacon wrote:
> On Sat, Feb 09, 2019 at 07:34:53PM +0100, AngeloGioacchino Del Regno wrote:
>> From 33fb6d036de273bb71ac1c67d7a91b7a5148e659 Mon Sep 17 00:00:00 2001
>> From: "Angelo G. Del Regno" <kholk11@gmail.com>
>> Date: Sat, 9 Feb 2019 18:56:46 +0100
>> Subject: [PATCH] arm64/io: Don't use WZR in writel
>>
>> This is a partial revert of commit ee5e41b5f21a
>> ("arm64/io: Allow I/O writes to use {W,X}ZR")
>>
>> When we try to use the zero register directly on some SoCs,
>> their security will make them freeze due to a firmware bug.>> This behavior is seen with the arm-smmu driver freezing on
>> TLBI and TLBSYNC on MSM8996, MSM8998, SDM630, SDM660.

This looks similar to the issue these SoCs have with GICv3, worked
around in 9c8114c20d18.

> Hmm, this sounds very fragile. I hope they're not trapping and emulating
> MMIO accesses and treating the zero register as the stack pointer...

I bet this is the case. The same bug was there in both KVM and Xen. The
only difference is that we fixed it back in December 2015 (at least for
KVM), while some of these SoCs were announced in 2017, and are still
shipping. Great stuff.

> 
> Wouldn't this also be triggerable from userspace by mmap()ing either
> /dev/mem or e.g. a PCI bar via sysfs?
> 
>> Allocating a temporary register to store the zero for the
>> write actually solves the issue on these SoCs.
> 
> I don't think this catches all MMIO accesses, so I think we need to
> understand more about the actual issue here. For example, is it only the
> SMMU that causes this problem? Also, any workaround should be specific to
> the broken SoCs.

Also, nothing would prevent a compiler from generating these accesses.

	M.

Jazz is not dead. It just smells funny...