Re: [PATCH 2/3] x86/mm/pti: Don't clear permissions in pti_clone_pmd()

[Andy Lutomirski]

On Tue, Aug 7, 2018 at 11:34 AM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> On 08/07/2018 03:24 AM, Joerg Roedel wrote:
>> The function sets the global-bit on cloned PMD entries,
>> which only makes sense when the permissions are identical
>> between the user and the kernel page-table.
>>
>> Further, only write-permissions are cleared for entry-text
>> and kernel-text sections, which are not writeable anyway.
>
> I think this patch is correct, but I'd be curious if Andy remembers why
> we chose to clear _PAGE_RW on these things.  It might have been that we
> were trying to say that the *entry* code shouldn't write to this stuff,
> regardless of whether the normal kernel can.
>
> But, either way, I agree with the logic here that Global pages must
> share permissions between both mappings, so feel free to add my Ack.  I
> just want to make sure Andy doesn't remember some detail I'm forgetting.

I suspect it's because we used to (and maybe still do) initialize the
user tables before mark_read_only().