Message-ID: <faf88ba7-cb9c-f7ec-07f5-e3971bd35a4c@kernel.org>
Date:   Thu, 2 Dec 2021 17:11:02 -0800
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Subject: Re: [RFC v2 PATCH 01/13] mm/shmem: Introduce F_SEAL_GUEST
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To:     Chao Peng <chao.p.peng@linux.intel.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org, qemu-devel@nongnu.org
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
        Hugh Dickins <hughd@google.com>,
        Jeff Layton <jlayton@kernel.org>,
        "J . Bruce Fields" <bfields@fieldses.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Yu Zhang <yu.c.zhang@linux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        john.ji@intel.com, susie.li@intel.com, jun.nakajima@intel.com,
        dave.hansen@intel.com, ak@linux.intel.com, david@redhat.com
References: <20211119134739.20218-1-chao.p.peng@linux.intel.com>
 <20211119134739.20218-2-chao.p.peng@linux.intel.com>
From:   Andy Lutomirski <luto@kernel.org>
In-Reply-To: <20211119134739.20218-2-chao.p.peng@linux.intel.com>
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On 11/19/21 05:47, Chao Peng wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.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.