linux-kernel - Re: [RFC v2 PATCH 01/13] mm/shmem: Introduce F_SEAL

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Message-ID: <20211122150956.GS876299@ziepe.ca>
Date:   Mon, 22 Nov 2021 11:09:56 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     David Hildenbrand <david@...hat.com>
Cc:     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, 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>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, john.ji@...el.com, susie.li@...el.com,
        jun.nakajima@...el.com, dave.hansen@...el.com, ak@...ux.intel.com
Subject: Re: [RFC v2 PATCH 01/13] mm/shmem: Introduce F_SEAL_GUEST

On Mon, Nov 22, 2021 at 03:57:17PM +0100, David Hildenbrand wrote:
> On 22.11.21 15:01, Jason Gunthorpe wrote:
> > On Mon, Nov 22, 2021 at 02:35:49PM +0100, David Hildenbrand wrote:
> >> On 22.11.21 14:31, Jason Gunthorpe wrote:
> >>> On Mon, Nov 22, 2021 at 10:26:12AM +0100, David Hildenbrand wrote:
> >>>
> >>>> I do wonder if we want to support sharing such memfds between processes
> >>>> in all cases ... we most certainly don't want to be able to share
> >>>> encrypted memory between VMs (I heard that the kernel has to forbid
> >>>> that). It would make sense in the use case you describe, though.
> >>>
> >>> If there is a F_SEAL_XX that blocks every kind of new access, who
> >>> cares if userspace passes the FD around or not?
> >> I was imagining that you actually would want to do some kind of "change
> >> ownership". But yeah, the intended semantics and all use cases we have
> >> in mind are not fully clear to me yet. If it's really "no new access"
> >> (side note: is "access" the right word?) then sure, we can pass the fd
> >> around.
> > 
> > What is "ownership" in a world with kvm and iommu are reading pages
> > out of the same fd?
> 
> In the world of encrypted memory / TDX, KVM somewhat "owns" that memory
> IMHO (for example, only it can migrate or swap out these pages; it's
> might be debatable if the TDX module or KVM actually "own" these pages ).

Sounds like it is a swap provider more than an owner?

Jason