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

[Andy Lutomirski]

On Tue, Aug 7, 2018 at 1:21 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> On Tue, 7 Aug 2018, Andy Lutomirski wrote:
>
>> 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().
>
> We still do that because we need the entry stuff working for interrupts
> early on. We now repeat the clone after mark_ro so the mask RW is not
> longer required.

Agreed.