Re: [PATCH v15 08/26] x86/mm: Introduce _PAGE_COW

Yu, Yu-cheng
2020-12-10T18:12:40Z

On 12/10/2020 9:41 AM, Borislav Petkov wrote:
> On Tue, Dec 08, 2020 at 11:24:16AM -0800, Yu, Yu-cheng wrote:
>> Case (a) is a normal writable data page that has gone through fork(). So it
>
> Writable >
>> has W=0, D=1. But here, the software chooses not to use the D bit, and
>
> But it has W=0. So not writable?

Maybe I will change to: A page in a writable vma, has been modified and
gone through fork().

>> instead, W=0, COW=1.
>
> So the "new" way of denoting that the page is modified is COW=1
> *when* on CET hw. The D=1 bit is still used on the rest thus the two
> _PAGE_DIRTY_BITS.
>
> Am I close?

COW=1 is only used in copy-on-write situation (when CET is enabled). If
W=1, D bit is used.

>> Case (b) is a normal read-only data page. Since it is read-only, fork()
>> won't affect it. In __get_user_pages(), a copy of the read-only page is
>> needed, and the page is duplicated. The software sets COW=1 for the new
>> copy.
>
> That makes more sense.
>
>> Thread-A is writing to a writable page, and the page's PTE is becoming W=1,
>> D=1. In the middle of it, Thread-B is changing the PTE to W=0.
>
> Yah, add that to the explanation pls.
>

Sure.