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

[Josh Poimboeuf]

On Wed, Sep 09, 2020 at 07:46:51AM -0500, Josh Poimboeuf wrote:
> On Wed, Sep 09, 2020 at 08:20:07AM +0000, David Laight wrote:
> > From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> > > 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?

Looks like the set_fs() removal patches are now in the vfs tree for
-next.  I doubt this is going in an -rc so I'll rework the patch based
on top of Christoph's changes.

-- 
Josh