linux-kernel - Re: [PATCH v5] x86: Enable fast strings on Intel if BIOS hasn't already

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Message-ID: <5181527A.8080904@zytor.com>
Date:	Wed, 01 May 2013 10:35:54 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Theodore Ts'o" <tytso@....edu>, Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...capital.net>,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	Andrew Lutomirski <luto@....edu>
Subject: Re: [PATCH v5] x86: Enable fast strings on Intel if BIOS hasn't already

On 05/01/2013 10:20 AM, Theodore Ts'o wrote:
> On Wed, May 01, 2013 at 09:42:30AM -0700, H. Peter Anvin wrote:
>> The erratum reads seriously, but it only affects crossings between pages
>> of different page types, which is rare in itself.  WT and WP are not
>> even used in Linux; the UC case we end up doing 8-byte stores instead of
>> the proper size, which is wrong, but for the case where the user is
>> malicious the user could just do that directly, and it seems extremely
>> hard to envision a scenario where someone would do that intentionally.
> 
> Yeah, I wasn't so much worried about a malicious user as much as a
> situation where the you're trying to debug a mysterious and
> hard-to-reproduce failure, start tearing your hair out, and wondering
> whether you're going insane or the compiler hates you and is out to
> get you and you start staring at assembly code to try to figure out
> how some piece of memory got mysteriously corrupted....
> 

If you are crossing pages with different memory types, the fact that the
sizes being written are wrong is probably the least of your problems.

	-hpa


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