linux-kernel - Re: [PATCH v2 2/2] drm/ttm: Fix vm page protection handling

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20191204143521.GJ25242@dhcp22.suse.cz>
Date:   Wed, 4 Dec 2019 15:35:21 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, pv-drivers@...are.com,
        linux-graphics-maintainer@...are.com,
        Thomas Hellstrom <thellstrom@...are.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Christian König <christian.koenig@....com>
Subject: Re: [PATCH v2 2/2] drm/ttm: Fix vm page protection handling

On Wed 04-12-19 15:16:09, Thomas Hellström (VMware) wrote:
> On 12/4/19 2:52 PM, Michal Hocko wrote:
> > On Tue 03-12-19 11:48:53, Thomas Hellström (VMware) wrote:
> > > From: Thomas Hellstrom <thellstrom@...are.com>
> > > 
> > > TTM graphics buffer objects may, transparently to user-space,  move
> > > between IO and system memory. When that happens, all PTEs pointing to the
> > > old location are zapped before the move and then faulted in again if
> > > needed. When that happens, the page protection caching mode- and
> > > encryption bits may change and be different from those of
> > > struct vm_area_struct::vm_page_prot.
> > > 
> > > We were using an ugly hack to set the page protection correctly.
> > > Fix that and instead use vmf_insert_mixed_prot() and / or
> > > vmf_insert_pfn_prot().
> > > Also get the default page protection from
> > > struct vm_area_struct::vm_page_prot rather than using vm_get_page_prot().
> > > This way we catch modifications done by the vm system for drivers that
> > > want write-notification.
> > So essentially this should have any new side effect on functionality it
> > is just making a hacky/ugly code less so?
> 
> Functionality is unchanged. The use of a on-stack vma copy was severely
> frowned upon in an earlier thread, which also points to another similar
> example using vmf_insert_pfn_prot().
> 
> https://lore.kernel.org/lkml/20190905103541.4161-2-thomas_os@shipmail.org/
> 
> > In other words what are the
> > consequences of having page protection inconsistent from vma's?
> 
> During the years, it looks like the caching- and encryption flags of
> vma::vm_page_prot have been largely removed from usage. From what I can
> tell, there are no more places left that can affect TTM. We discussed
> __split_huge_pmd_locked() towards the end of that thread, but that doesn't
> affect TTM even with huge page-table entries.

Please state all those details/assumptions you are operating on in the
changelog.
-- 
Michal Hocko
SUSE Labs