linux-kernel - Re: [RESENT PATCH] x86/mem: fix the offset overflow when read/write mem

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Message-ID: <1494344803.20270.27.camel@redhat.com>
Date:   Tue, 09 May 2017 11:46:43 -0400
From:   Rik van Riel <riel@...hat.com>
To:     zhong jiang <zhongjiang@...wei.com>
Cc:     David Rientjes <rientjes@...gle.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        Andrew Morton <akpm@...ux-foundation.org>, arnd@...db.de,
        hannes@...xchg.org, kirill@...temov.name,
        mgorman@...hsingularity.net, hughd@...gle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Xishi Qiu <qiuxishi@...wei.com>
Subject: Re: [RESENT PATCH] x86/mem: fix the offset overflow when read/write
 mem

On Thu, 2017-05-04 at 10:28 +0800, zhong jiang wrote:
> On 2017/5/4 2:46, Rik van Riel wrote:

> > However, it is not as easy as simply checking the
> > end against __pa(high_memory). Some systems have
> > non-contiguous physical memory ranges, with gaps
> > of invalid addresses in-between.
> 
>  The invalid physical address means that it is used as
>  io mapped. not in system ram region. /dev/mem is not
>  access to them , is it right?

Not necessarily. Some systems simply have large
gaps in physical memory access. Their memory map
may look like this:

|MMMMMM|IO|MMMM|..................|MMMMMMMM|

Where M is memory, IO is IO space, and the
dots are simply a gap in physical address
space with no valid accesses at all.

> > At that point, is the complexity so much that it no
> > longer makes sense to try to protect against root
> > crashing the system?
> > 
> 
>  your suggestion is to let the issue along without any protection.
>  just root user know what they are doing.

Well, root already has other ways to crash the system.

Implementing validation on /dev/mem may make sense if
it can be done in a simple way, but may not be worth
it if it becomes too complex.

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