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

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Date:   Wed, 9 Sep 2020 08:20:07 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Josh Poimboeuf' <jpoimboe@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>
CC:     "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 <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.

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

While it may be possible to fake 'asm volatile goto with outputs'
by using a local asm register variable and alternative pattern
might be to have access_ok() return 0 (fail) or ~0 (ok).
Then the usage can be (with a load of casts):
	p1 = p & access_ok(p);
	if (!p1 && p)
		return -EFAULT;
	foo(*p1);
With any luck the compiler will use the result of the & for the
!p1 test.

	David

	

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