linux-kernel - Re: [PATCH 1/5] mm: Introduce mm_struct.has

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Date:   Thu, 24 Sep 2020 11:02:37 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Jan Kara <jack@...e.cz>
Cc:     Peter Xu <peterx@...hat.com>, John Hubbard <jhubbard@...dia.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>,
        Christoph Hellwig <hch@....de>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned

On Thu, Sep 24, 2020 at 09:44:09AM +0200, Jan Kara wrote:
> > After the page is pinned it is prevented from being freed and
> > recycled. After GUP has the pin it must check that the PTE still
> > points at the same page, otherwise it might have pinned a page that is
> > alreay free'd - and that would be a use-after-free issue.
> 
> I don't think a page use-after-free is really the reason - we add page
> reference through page_ref_add_unless(page, x, 0) - i.e., it will fail for
> already freed page. 

I mean, the page could have been freed and already reallocated with a
positive refcount, so the add_unless check isn't protective.

The add_unless prevents the page from being freed. The 2nd pte read
ensures it wasn't already freed/reassigned before the pin.

If something drives the page refcount to zero then it is already
synchronized with GUP fast because of the atomic add_unless, no need
to re-check the pte for that case?? But I don't know what the DAX case
is you mentioned.

Jason