Re: [PATCHv3 5/8] x86/uaccess: Provide untagged_addr() and remove tags before address check

Kirill A. Shutemov
2022-06-16T16:49:33Z

On Thu, Jun 16, 2022 at 12:02:16PM +0200, Peter Zijlstra wrote:
> On Fri, Jun 10, 2022 at 05:35:24PM +0300, Kirill A. Shutemov wrote:
> > +#ifdef CONFIG_X86_64
> > +/*
> > + * Mask out tag bits from the address.
> > + *
> > + * Magic with the 'sign' allows to untag userspace pointer without any branches
> > + * while leaving kernel addresses intact.
> > + */
> > +#define untagged_addr(mm, addr) ({ \
> > + u64 __addr = (__force u64)(addr); \
> > + s64 sign = (s64)__addr >> 63; \
> > + __addr ^= sign; \
> > + __addr &= (mm)->context.untag_mask; \
> > + __addr ^= sign; \
> > + (__force __typeof__(addr))__addr; \
> > +})
>
> Can't we make that mask a constant and *always* unmask U57 irrespective
> of LAM being on?

We can do this if we give up on LAM_U48.

It would also needlessly relax canonical check. I'm not sure it is a good
idea.

--
Kirill A. Shutemov