linux-kernel - Re: [PATCH v3 1/4] mm: introduce get_user_pages

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: <20171204093156.mp36zkcwrxkenixb@dhcp22.suse.cz>
Date:   Mon, 4 Dec 2017 10:31:56 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        linux-rdma <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH v3 1/4] mm: introduce get_user_pages_longterm

On Fri 01-12-17 08:29:53, Dan Williams wrote:
> On Fri, Dec 1, 2017 at 8:02 AM, Jason Gunthorpe <jgg@...pe.ca> wrote:
> >
> > On Fri, Dec 01, 2017 at 11:12:18AM +0100, Michal Hocko wrote:
> > > On Thu 30-11-17 12:01:17, Jason Gunthorpe wrote:
> > > > On Thu, Nov 30, 2017 at 10:32:42AM -0800, Dan Williams wrote:
> > > > > > Who and how many LRU pages can pin that way and how do you prevent nasty
> > > > > > users to DoS systems this way?
> > > > >
> > > > > I assume this is something the RDMA community has had to contend with?
> > > > > I'm not an RDMA person, I'm just here to fix dax.
> > > >
> > > > The RDMA implementation respects the mlock rlimit
> > >
> > > OK, so then I am kind of lost in why do we need a special g-u-p variant.
> > > The documentation doesn't say and quite contrary it assumes that the
> > > caller knows what he is doing. This cannot be the right approach.
> >
> > I thought it was because get_user_pages_longterm is supposed to fail
> > on DAX mappings?
> 
> Correct, the rlimit checks are a separate issue,
> get_user_pages_longterm is only there to avoid open coding vma lookup
> and vma_is_fsdax() checks in multiple code paths.

Then it is a terrible misnomer. One would expect this is a proper way to
get a longterm pin on a page.
 
> > And maybe we should think about moving the rlimit accounting into this
> > new function too someday?
> 
> DAX pages are not accounted in any rlimit because they are statically
> allocated reserved memory regions.

Which is OK, but how do you prevent anybody calling this function on
normal LRU pages?

-- 
Michal Hocko
SUSE Labs