Re: [PATCH v3 kvm/queue 01/16] mm/shmem: Introduce F_SEAL_INACCESSIBLE

[David Hildenbrand <david@redhat.com>]

On 23.12.21 13:29, Chao Peng wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> 
> Introduce a new seal F_SEAL_INACCESSIBLE indicating the content of
> the file is inaccessible from userspace in any possible ways like
> read(),write() or mmap() etc.
> 
> It provides semantics required for KVM guest private memory support
> that a file descriptor with this seal set is going to be used as the
> source of guest memory in confidential computing environments such
> as Intel TDX/AMD SEV but may not be accessible from host userspace.
> 
> At this time only shmem implements this seal.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> ---
>  include/uapi/linux/fcntl.h |  1 +
>  mm/shmem.c                 | 37 +++++++++++++++++++++++++++++++++++--
>  2 files changed, 36 insertions(+), 2 deletions(-)
> 
> diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
> index 2f86b2ad6d7e..e2bad051936f 100644
> --- a/include/uapi/linux/fcntl.h
> +++ b/include/uapi/linux/fcntl.h
> @@ -43,6 +43,7 @@
>  #define F_SEAL_GROW	0x0004	/* prevent file from growing */
>  #define F_SEAL_WRITE	0x0008	/* prevent writes */
>  #define F_SEAL_FUTURE_WRITE	0x0010  /* prevent future writes while mapped */
> +#define F_SEAL_INACCESSIBLE	0x0020  /* prevent file from accessing */

I think this needs more clarification: the file content can still be
accessed using in-kernel mechanisms such as MEMFD_OPS for KVM. It
effectively disallows traditional access to a file (read/write/mmap)
that will result in ordinary MMU access to file content.

Not sure how to best clarify that: maybe, prevent ordinary MMU access
(e.g., read/write/mmap) to file content?

>  /* (1U << 31) is reserved for signed error codes */
>  
>  /*
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 18f93c2d68f1..faa7e9b1b9bc 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1098,6 +1098,10 @@ static int shmem_setattr(struct user_namespace *mnt_userns,
>  		    (newsize > oldsize && (info->seals & F_SEAL_GROW)))
>  			return -EPERM;
>  
> +		if ((info->seals & F_SEAL_INACCESSIBLE) &&
> +		    (newsize & ~PAGE_MASK))
> +			return -EINVAL;
> +

What happens when sealing and there are existing mmaps?


-- 
Thanks,

David / dhildenb