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

Kirill A. Shutemov
2022-06-15T17:01:36Z

On Mon, Jun 13, 2022 at 05:36:43PM +0000, Edgecombe, Rick P wrote:
> On Fri, 2022-06-10 at 17:35 +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.
>
> Trying to understand the magic part here. I guess how it works is, when
> the high bit is set, it does the opposite of untagging the addresses by
> setting the tag bits instead of clearing them. So:
> - For proper canonical kernel addresses (with U57) it leaves them
> intact since the tag bits were already set.
> - For non-canonical kernel-half addresses, it fixes them up.
> (0xeffffff000000840->0xfffffff000000840)
> - For U48 and 5 level paging, it corrupts some normal kernel
> addresses. (0xff90ffffffffffff->0xffffffffffffffff)
>
> I just ported this to userspace and threw some addresses at it to see
> what happened, so hopefully I got that right.

Ouch. Thanks for noticing this. I should have catched this myself. Yes,
this implementation is broken for LAM_U48 on 5-level machine.

What about this:

#define untagged_addr(mm, addr) ({ \
u64 __addr = (__force u64)(addr); \
s64 sign = (s64)__addr >> 63; \
__addr &= (mm)->context.untag_mask | sign; \
(__force __typeof__(addr))__addr; \
})

It makes mask effectively. all-ones for supervisor addresses. And it is
less magic to my eyes.

The generated code also look sane to me:

11d0: 48 89 f8 mov %rdi,%rax
11d3: 48 c1 f8 3f sar $0x3f,%rax
11d7: 48 0b 05 52 2e 00 00 or 0x2e52(%rip),%rax # 4030 <untag_mask>
11de: 48 21 f8 and %rdi,%rax

Any comments?

> Is this special kernel address handling only needed because
> copy_to_kernel_nofault(), etc call the user helpers?

I did not have any particular use-case in mind. But just if some kernel
address gets there and bits get cleared we will have very hard to debug
bug.

--
Kirill A. Shutemov