Re: On the correctness of dbe3ed1c078c193be34326728d494c5c4bc115e2

[H. Peter Anvin]

On 09/01/2013 08:58 AM, Linus Torvalds wrote:
> On Sun, Sep 1, 2013 at 5:20 AM, H. Peter Anvin <hpa@xxxxxxxxx> 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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