Re: [PATCH 09/30] x86, kaiser: only populate shadow page tables for userspace

[Andy Lutomirski]

On Tue, Nov 21, 2017 at 2:09 PM, Dave Hansen
<dave.hansen@xxxxxxxxxxxxxxx> wrote:
> On 11/20/2017 12:12 PM, Thomas Gleixner wrote:
>>> +                     */
>>> +                    native_get_shadow_pgd(pgdp)->pgd = pgd.pgd;
>>> +                    /*
>>> +                     * For the copy of the pgd that the kernel
>>> +                     * uses, make it unusable to userspace.  This
>>> +                     * ensures if we get out to userspace with the
>>> +                     * wrong CR3 value, userspace will crash
>>> +                     * instead of running.
>>> +                     */
>>> +                    pgd.pgd |= _PAGE_NX;
>>> +            }
>>> +    } else if (!pgd.pgd) {
>>> +            /*
>>> +             * We are clearing the PGD and can not check  _PAGE_USER
>>> +             * in the zero'd PGD.
>>
>> Just the argument cannot be checked because it's clearing the entry. The
>> pgd entry itself is not yet modified, so it could be checked.
>
> So, I guess we could enforce that only PGDs with _PAGE_USER set can ever
> be cleared.  That has a nice symmetry to it because we set the shadow
> when we see _PAGE_USER and we would then clear the shadow when we see
> _PAGE_USER.

Is this code path ever hit in any case other than tearing down an LDT?

I'm tempted to suggest that KAISER just disable the MODIFY_LDT config
option for now...