Re: [RFC v2 PATCH 01/13] mm/shmem: Introduce F_SEAL_GUEST

[Andy Lutomirski]

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.