linux-kernel - Re: [PATCH v2 7/7] ARM: mm: Fix PXN process with LPAE feature

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Message-ID: <20210602155843.GN30436@shell.armlinux.org.uk>
Date:   Wed, 2 Jun 2021 16:58:43 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Kefeng Wang <wangkefeng.wang@...wei.com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jungseung Lee <js07.lee@...il.com>
Subject: Re: [PATCH v2 7/7] ARM: mm: Fix PXN process with LPAE feature

On Wed, Jun 02, 2021 at 11:13:14PM +0800, Kefeng Wang wrote:
>   IFSR format when using the Short-descriptor translation table format
> 
>     Domain fault      01001            First level   01011     Second level
> 
>     Permission fault 01101            First level   01111     Second level
> 
>   IFSR format when using the Long-descriptor translation table format
> 
>    0011LL Permission fault. LL bits indicate levelb.
> 
> After check the ARM spec, I think for the permission fault,  we should panic
> with or without LPAE, will change to

As I explained in one of the previous patches, the page tables that get
used for mapping kernel space are the _tasks_ own page tables. Any new
kernel mappings are lazily copied to the task page tables - such as
when a module is loaded.

The first time we touch a page, we could end up with a page translation
fault. This will call do_page_fault(), and so with your proposal,
loading a module will potentially cause a kernel panic in this case,
probably leading to systems that panic early during userspace boot.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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