linux-kernel - Re: [PATCH v3] MIPS: kernel: Reserve exception base early to prevent corruption

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Message-ID: <20210308190057.GA15111@alpha.franken.de>
Date:   Mon, 8 Mar 2021 20:00:57 +0100
From:   Thomas Bogendoerfer <tsbogend@...ha.franken.de>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Mike Rapoport <rppt@...nel.org>, Roman Gushchin <guro@...com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        Kamal Dasu <kdasu.kdev@...il.com>,
        Serge Semin <Sergey.Semin@...kalelectronics.ru>
Subject: Re: [PATCH v3] MIPS: kernel: Reserve exception base early to prevent
 corruption

On Mon, Mar 08, 2021 at 10:21:10AM -0800, Florian Fainelli wrote:
> On 3/8/21 1:24 AM, Thomas Bogendoerfer wrote:
> > BMIPS is one of the few platforms that do change the exception base.
> > After commit 2dcb39645441 ("memblock: do not start bottom-up allocations
> > with kernel_end") we started seeing BMIPS boards fail to boot with the
> > built-in FDT being corrupted.
> > 
> > Before the cited commit, early allocations would be in the [kernel_end,
> > RAM_END] range, but after commit they would be within [RAM_START +
> > PAGE_SIZE, RAM_END].
> > 
> > The custom exception base handler that is installed by
> > bmips_ebase_setup() done for BMIPS5000 CPUs ends-up trampling on the
> > memory region allocated by unflatten_and_copy_device_tree() thus
> > corrupting the FDT used by the kernel.
> > 
> > To fix this, we need to perform an early reservation of the custom
> > exception space. Additional we reserve the first 4k (1k for R3k) for
> > either normal exception vector space (legacy CPUs) or special vectors
> > like cache exceptions.
> > 
> > Huge thanks to Serge for analysing and proposing a solution to this
> > issue.
> > 
> > Fixes: 2dcb39645441 ("memblock: do not start bottom-up allocations with kernel_end")
> > Reported-by: Kamal Dasu <kdasu.kdev@...il.com>
> > Debugged-by: Serge Semin <Sergey.Semin@...kalelectronics.ru>
> > Signed-off-by: Thomas Bogendoerfer <tsbogend@...ha.franken.de>
> > ---
> > Changes in v3:
> >  - always reserve the first 4k for all CPUs (1k for R3k)
> > 
> > Changes in v2:
> >  - do only memblock reservation in reserve_exception_space()
> >  - reserve 0..0x400 for all CPUs without ebase register and
> >    to addtional reserve_exception_space for BMIPS CPUs
> 
> Thomas, do you mind CC'ing me for subsequent versions so you can get a
> chance to have a Tested-by tag? Thank you!

sure, I hope it's the last version ;-)

> Tested-by: Florian Fainelli <f.fainelli@...il.com>

thank you.

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.                                                [ RFC1925, 2.3 ]