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

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Date:   Wed, 9 Sep 2020 13:46:24 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Josh Poimboeuf' <jpoimboe@...hat.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

From: Josh Poimboeuf
> Sent: 09 September 2020 13:47
> 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 think that has already all been removed from x86.

	David

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