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

[Dave Hansen]

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.