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

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Message-ID: <20211122140148.GR876299@ziepe.ca>
Date:   Mon, 22 Nov 2021 10:01:48 -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 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?

"no new access" makes sense to me, we have access through
read/write/mmap/splice/etc and access to pages through the private in
kernel interface (kvm, iommu)

Jason