linux-kernel - Re: stop messing with set_fs in arm

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Date:   Tue, 14 Apr 2020 16:59:16 +0100
From:   James Morse <james.morse@....com>
To:     Christoph Hellwig <hch@....de>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: stop messing with set_fs in arm_sdei

Hi Christoph,

On 14/04/2020 15:23, Christoph Hellwig wrote:
> can you take a look at this series?  I've been trying to figure out
> what the set_fs in arm_sdei is good for, and could not find any
> good reason.  But I don't have any hardware implementing this interface,
> so the changes are entirely untested.

Its a firmware thing, think of it as a firmware assisted software NMI.

The arch code save/restores set_fs() because the entry code does that when taking an
exception from EL1. SDEI does the same because it doesn't come via the same entry code. It
does it in C because that C is always run before the handler, something that isn't true
for the regular assembly version.

The regular entry code does this because any exception may have interrupted code that had
addr_limit set to something else:
https://bugs.chromium.org/p/project-zero/issues/detail?id=822

and the patch that fixed it: commit e19a6ee2460b "arm64: kernel: Save and restore UAO and
addr_limit on exception entry"

Thanks,

James