linux-kernel - Re: [x86/asm/entry] BUG: unable to handle kernel paging request

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Date:	Mon, 9 Mar 2015 15:08:35 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Fengguang Wu <fengguang.wu@...el.com>, X86 ML <x86@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>, LKP <lkp@...org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [x86/asm/entry] BUG: unable to handle kernel paging request

On Mon, Mar 9, 2015 at 3:00 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> I'm preparing a patch which does this:
>>
>>         btl     $X86_EFLAGS_VM_BIT,PT_EFLAGS(%esp)
>>         jc      restore_nocheck         # VM set, not it
>>         testb   $3,PT_CS(%esp)
>>         jz      restore_nocheck         # CPL0, not it
>>         # Note: we access PT_OLDSS only when we know it exists.
>>         # If PT_CS is from CPL0, this can be not true.
>>         testb   $SEGMENT_TI_MASK,PT_OLDSS(%esp)
>>         jnz     ldt_ss                  # returning to user-space with LDT SS
>>
>> All three checks can run in parallel on an OOO CPU.
>> Most of the time, none of branches will be taken.
>
> Why do we need the VM check?  Isn't CS == 0 in vm86 mode?

IF VM is set, then we are in vm86 mode, not in 16-bit protected mode
where ESPFIX thingy may be necessary. And also,
%ss register in vm86 does nit have LDT bit, thus that check would
not work properly anyway.
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