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

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Date:   Thu, 2 Dec 2021 17:11:02 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     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
Cc:     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>,
        john.ji@...el.com, susie.li@...el.com, jun.nakajima@...el.com,
        dave.hansen@...el.com, ak@...ux.intel.com, david@...hat.com
Subject: Re: [RFC v2 PATCH 01/13] mm/shmem: Introduce F_SEAL_GUEST

On 11/19/21 05:47, Chao Peng wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> 
> The new seal type provides semantics required for KVM guest private
> memory support. A file descriptor with the seal set is going to be used
> as source of guest memory in confidential computing environments such as
> Intel TDX and AMD SEV.
> 
> F_SEAL_GUEST can only be set on empty memfd. After the seal is set
> userspace cannot read, write or mmap the memfd.

I don't have a strong objection here, but, given that you're only 
supporting it for memfd, would a memfd_create() flag be more 
straightforward?  If nothing else, it would avoid any possible locking 
issue.

I'm also very very slightly nervous about a situation in which one 
program sends a memfd to an untrusted other process and that process 
truncates the memfd and then F_SEAL_GUESTs it.  This could be mostly 
mitigated by also requiring that no other seals be set when F_SEAL_GUEST 
happens, but the alternative MFD_GUEST would eliminate this issue too.