linux-kernel - Re: [PATCH v9 1/8] mm: Introduce memfd_restricted system call to create restricted user memory

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: <20221114152843.ylxe4dis254vrj5u@box.shutemov.name>
Date:   Mon, 14 Nov 2022 18:28:43 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Michael Roth <michael.roth@....com>,
        Chao Peng <chao.p.peng@...ux.intel.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-api@...r.kernel.org, linux-doc@...r.kernel.org,
        qemu-devel@...gnu.org, Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>, Mike Rapoport <rppt@...nel.org>,
        Steven Price <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, jun.nakajima@...el.com, dave.hansen@...el.com,
        ak@...ux.intel.com, david@...hat.com, aarcange@...hat.com,
        ddutile@...hat.com, dhildenb@...hat.com,
        Quentin Perret <qperret@...gle.com>, tabba@...gle.com,
        mhocko@...e.com, Muchun Song <songmuchun@...edance.com>,
        wei.w.wang@...el.com
Subject: Re: [PATCH v9 1/8] mm: Introduce memfd_restricted system call to
 create restricted user memory

On Mon, Nov 14, 2022 at 03:02:37PM +0100, Vlastimil Babka wrote:
> On 11/1/22 16:19, Michael Roth wrote:
> > On Tue, Nov 01, 2022 at 07:37:29PM +0800, Chao Peng wrote:
> >> > 
> >> >   1) restoring kernel directmap:
> >> > 
> >> >      Currently SNP (and I believe TDX) need to either split or remove kernel
> >> >      direct mappings for restricted PFNs, since there is no guarantee that
> >> >      other PFNs within a 2MB range won't be used for non-restricted
> >> >      (which will cause an RMP #PF in the case of SNP since the 2MB
> >> >      mapping overlaps with guest-owned pages)
> >> 
> >> Has the splitting and restoring been a well-discussed direction? I'm
> >> just curious whether there is other options to solve this issue.
> > 
> > For SNP it's been discussed for quite some time, and either splitting or
> > removing private entries from directmap are the well-discussed way I'm
> > aware of to avoid RMP violations due to some other kernel process using
> > a 2MB mapping to access shared memory if there are private pages that
> > happen to be within that range.
> > 
> > In both cases the issue of how to restore directmap as 2M becomes a
> > problem.
> > 
> > I was also under the impression TDX had similar requirements. If so,
> > do you know what the plan is for handling this for TDX?
> > 
> > There are also 2 potential alternatives I'm aware of, but these haven't
> > been discussed in much detail AFAIK:
> > 
> > a) Ensure confidential guests are backed by 2MB pages. shmem has a way to
> >    request 2MB THP pages, but I'm not sure how reliably we can guarantee
> >    that enough THPs are available, so if we went that route we'd probably
> >    be better off requiring the use of hugetlbfs as the backing store. But
> >    obviously that's a bit limiting and it would be nice to have the option
> >    of using normal pages as well. One nice thing with invalidation
> >    scheme proposed here is that this would "Just Work" if implement
> >    hugetlbfs support, so an admin that doesn't want any directmap
> >    splitting has this option available, otherwise it's done as a
> >    best-effort.
> > 
> > b) Implement general support for restoring directmap as 2M even when
> >    subpages might be in use by other kernel threads. This would be the
> >    most flexible approach since it requires no special handling during
> >    invalidations, but I think it's only possible if all the CPA
> >    attributes for the 2M range are the same at the time the mapping is
> >    restored/unsplit, so some potential locking issues there and still
> >    chance for splitting directmap over time.
> 
> I've been hoping that
> 
> c) using a mechanism such as [1] [2] where the goal is to group together
> these small allocations that need to increase directmap granularity so
> maximum number of large mappings are preserved.

As I mentioned in the other thread the restricted memfd can be backed by
secretmem instead of plain memfd. It already handles directmap with care.

But I don't think it has to be part of initial restricted memfd
implementation. It is SEV-specific requirement and AMD folks can extend
implementation as needed later.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov