linux-kernel - Re: [tip:x86/asm] x86/asm/entry: Add user_mode_ignore

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Message-ID: <55155F9E.3030605@redhat.com>
Date:	Fri, 27 Mar 2015 14:48:14 +0100
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...nel.org>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Brad Spengler <spender@...ecurity.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Lutomirski <luto@...nel.org>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/asm] x86/asm/entry: Add user_mode_ignore_vm86()

On 03/24/2015 08:46 PM, Andy Lutomirski wrote:
> On Tue, Mar 24, 2015 at 12:44 PM, Ingo Molnar <mingo@...nel.org> wrote:
>>
>> * Andy Lutomirski <luto@...capital.net> wrote:
>>
>>> On Mon, Mar 23, 2015 at 5:26 AM, tip-bot for Andy Lutomirski
>>> <tipbot@...or.com> wrote:
>>>> Commit-ID:  a67e7277d01ccfd39b0db5a198c2643cc19dd79c
>>>> Gitweb:     http://git.kernel.org/tip/a67e7277d01ccfd39b0db5a198c2643cc19dd79c
>>>> Author:     Andy Lutomirski <luto@...nel.org>
>>>> AuthorDate: Wed, 18 Mar 2015 18:33:29 -0700
>>>> Committer:  Ingo Molnar <mingo@...nel.org>
>>>> CommitDate: Mon, 23 Mar 2015 11:13:36 +0100
>>>>
>>>> x86/asm/entry: Add user_mode_ignore_vm86()
>>>>
>>>> user_mode() is dangerous and user_mode_vm() has a confusing name.
>>>>
>>>> Add user_mode_ignore_vm86() (equivalent to current user_mode()).
>>>> We'll change the small number of legitimate users of user_mode()
>>>> to user_mode_ignore_vm86().
>>>>
>>>> Inspired by grsec, although this works rather differently.
>>>
>>> Ingo, does this mean that you changed your mind or do you still want
>>> a patch to delete user_mode_ignore_vm86 and just use user_mode
>>> everywhere instead?
>>
>> Would be still nice to have it as an add on patch, if you agree with
>> my arguments.
> 
> Given that there are only a very small number of callers left and
> they're all Obviously Correct (tm), I'm not too worried about it.
> Maybe if we kill off __copy_to_user, I'll be inspired to kill off
> user_mode_ignore_vm86 as well :)


I was looking at the code involving this function and it looks
like a much better name for user_mode_ignore_vm86() would be
user_mode_cs().

Every time we use it, we check vm8086 mode just before it:

perf_event.c

        if (regs->flags & X86_VM_MASK)
                return 0x10 * regs->cs;

        if (user_mode_ignore_vm86(regs) && regs->cs != __USER_CS)
                return get_segment_base(regs->cs);


traps.c (three similar instances):

        if (v8086_mode(regs)) {
...
                goto exit;
        }
        if (user_mode_ignore_vm86(regs))...


"_ignore_vm86" part doesn't quite work as an explanation.
user_mode_cs() would immediately tell me "do we have a user's cs?"

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