linux-kernel - Re: [kernel-hardening] [RFC 3/6] lib: vsprintf: physical address kernel pointer filtering options

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Date:   Sat, 06 May 2017 11:48:36 +0100
From:   Ian Campbell <ijc@...lion.org.uk>
To:     Greg KH <gregkh@...uxfoundation.org>,
        kernel-hardening@...ts.openwall.com,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        William Roberts <william.c.roberts@...el.com>,
        Chris Fries <cfries@...gle.com>,
        Dave Weinstein <olorin@...gle.com>
Subject: Re: [kernel-hardening] [RFC 3/6] lib: vsprintf: physical address
 kernel pointer filtering options

On Fri, 2017-05-05 at 21:07 -0700, Greg KH wrote:
> From: Dave Weinstein <olorin@...gle.com>
> 
> Add the kptr_restrict setting of 4 which results in %pa and
> %p[rR] values being replaced by zeros.

Given that '%pa' is:
 * - 'a[pd]' For address types [p] phys_addr_t, [d] dma_addr_t and derivatives
 *           (default assumed to be phys_addr_t, passed by reference)

what is the thread model which hiding physical addresses from attackers
protects against? I can see why virtual addresses would be obviously
dangerous but physical addresses seem less obvious and I didn't see it
spelled out in any of the commit messages or added comments in the
thread.

I think a comment somewhere would be useful for people who are trying
to decide if they should use %pa vs %paP etc.

Ian.