linux-kernel - Re: On the correctness of dbe3ed1c078c193be34326728d494c5c4bc115e2

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Message-ID: <522366F5.5080002@zytor.com>
Date:	Sun, 01 Sep 2013 09:10:29 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Randy Dunlap <rdunlap@...otime.net>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: On the correctness of dbe3ed1c078c193be34326728d494c5c4bc115e2

On 09/01/2013 08:58 AM, Linus Torvalds wrote:
> On Sun, Sep 1, 2013 at 5:20 AM, H. Peter Anvin <hpa@...or.com> wrote:
>>
>> This has the end result that we treat a user space instruction which
>> touches a privileged data structure that then page faults (e.g. a
>> segment load which causes #PF on the GDT) as a user-space fault.
>>
>> This seems very wrong to me, since such a #PF would indicate a serious
>> error in the kernel.
> 
> Not necessarily. Don't we basically do exactly that for the F00F bug
> workaround, for example?
> 
>                 Linus
> 

Actually, from looking at it, the F00F workaround is broken *exactly*
because of this patch.  By forcing PF_USER to set, we go into the
if (error_code & PF_USER) branch of __bad_area_semaphore(), which means
we *don't* do the F00F checking, and will deliver a SIGSEGV with the IDT
address to the user space process instead of SIGILL.

	-hpa

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