linux-kernel - Re: [PATCH v2] x86/uaccess: Use pointer masking to limit uaccess speculation

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Date:   Wed, 9 Sep 2020 07:46:44 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Will Deacon <will@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andy Lutomirski <luto@...nel.org>,
        Christoph Hellwig <hch@....de>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v2] x86/uaccess: Use pointer masking to limit uaccess
 speculation

On Wed, Sep 09, 2020 at 08:20:07AM +0000, David Laight wrote:
> From: Josh Poimboeuf <jpoimboe@...hat.com>
> > Sent: 08 September 2020 18:43
> > Hi x86 maintainers,
> ...
> > > --- a/arch/x86/lib/putuser.S
> > > +++ b/arch/x86/lib/putuser.S
> > > @@ -38,6 +38,8 @@ SYM_FUNC_START(__put_user_1)
> > >  	ENTER
> > >  	cmp TASK_addr_limit(%_ASM_BX),%_ASM_CX
> > >  	jae .Lbad_put_user
> > > +	sbb %_ASM_BX, %_ASM_BX		/* uaccess_mask_ptr() */
> > > +	and %_ASM_BX, %_ASM_CX
> > >  	ASM_STAC
> > >  1:	movb %al,(%_ASM_CX)
> > >  	xor %eax,%eax
> 
> For 64bit the sbb+and pattern can be replaced by an instruction
> that clears the high bit (eg btr $63, %rcx).
> This isn't dependant on the earlier instructions so can execute
> in parallel with them.

Wouldn't that break with KERNEL_DS?

> I still think that doing the same comparisons in access_ok()
> and for the pointer masking is silly - and they should get merged.

I agree, but as I said multiple times already, that will be a much
bigger change (tree-wide and multi-arch) which comes later.

-- 
Josh