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

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Message-ID: <55D61AF3.4090004@zytor.com>
Date:	Thu, 20 Aug 2015 11:22:43 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <bp@...e.de>, yalin wang <yalin.wang2010@...il.com>
CC:	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 08/16/2015 09:16 PM, Borislav Petkov wrote:
> On Mon, Aug 17, 2015 at 11:27:01AM +0800, yalin wang wrote:
>> i just want the x86 copy_from{to,in}_user() function have 
>> the same behaviour as other platforms.
> 
> Back to the original question from 2 mails ago:
> 
> How else would we be able to use the same function in copy_to and
> copy_from variants?
> 
>> and can disclose potential BUGs in kernel, if do like this.
> 
> Back to my other question:
> 
> Do you have any real life examples where you can trigger such bugs or is
> this only "potential"?
> 
> IOW, what I *think* you're trying to do sounds to me like unnecessary
> complication with no apparent gain *at* *all*. So show me why you want
> to do it: code it up, trigger a bug and show me why your version is
> better. No "but but it might be a good idea", no "potentially maybe",
> none of that maybe stuff. Write it, send it with instructions how
> someone else can apply it and trigger the issue. Ok?
> 

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.

The /proc/kcore issue is a completely different ball of wax, however.

	-hpa

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