linux-kernel - Re: [x86] copy_from{to}

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Message-ID: <55D792C8.6090006@zytor.com>
Date:	Fri, 21 Aug 2015 14:06:16 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <bp@...e.de>
CC:	yalin wang <yalin.wang2010@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
	x86@...nel.org, open list <linux-kernel@...r.kernel.org>,
	Will Deacon <will.deacon@....com>
Subject: Re: [x86] copy_from{to}_user question

On 08/20/2015 09:35 PM, Borislav Petkov wrote:
> On Thu, Aug 20, 2015 at 11:22:43AM -0700, H. Peter Anvin wrote:
>> There is a valid reason to do this, which is that currently
>> copy_{to,from}_user() effectively bypass SMAP as they don't verify that
>> the kernel pointer is actually a kernel pointer.
> 
> Well, we do STAC before we copy but SMAP is checking for supervisor
> access to *user* data. But you say "kernel pointers" which is supervisor
> data. What am I missing?
> 

What I'm saying is that we do do STAC, which *disables* SMAP.  We have
to do that because one pointer is known to be a user space pointer.

However, we currently don't verify that the *other* pointer is kernel
space, which it is supposed to be (if not, we should be using
copy_in_user).  We have to do this manually since we have to STAC which
means SMAP doesn't do anything at all.  I believe it would be a good
idea to add such checks (and they would even benefit non-SMAP hardware.)

	-hpa

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