Re: [PATCH 00/52] [RFC] virtio-fs: shared file system for virtual machines

[Vivek Goyal]

On Wed, Dec 12, 2018 at 06:07:40PM +0100, Piotr Jurkiewicz wrote:
> Currently, virtio-9p cannot be used with overlayfs in order to obtain
> Docker-like experience (but with separate kernel) because of file attributes
> problems. I wrote an email about that to qemu-devel almost year ago, but it
> received no attention (I attach its contents below.).
> 
> Will virtio-fs avoid these problems? I assume it will be transparent from
> the point of view of file attributes, and not enforce any kind of security
> filtering?

Hi Piotr,

So you want to use virtio-fs as upper/ layer of a overlay filesystem
inside guest? Interesting. I have not tried that.

As of now I think we are not doing any filtering of file attributes
and it might just work. Give it a try. Having said that, I suspect
that security model of virtio-fs most likely will evolve.

Thanks
Vivek
> 
> Piotr Jurkiewicz
> 
> ----
> 
> 1. Upper filesystem must support the creation of trusted.* extended
> attributes.
> 
> 9pfs has support for getting/setting xattrs, but calls operating on
> attributes other than user.* and system.posix_acl_* are dropped.
> 
> 2. Upper filesystem must provide valid d_type in readdir responses.
> 
> This works, but only in case of 'passtrough' and 'none' security models. In
> the case of 'mapped-xattr' and 'mapped-file' models, d_type is being zeroed
> to DT_UNKNOWN during readdir() call.
> 
> All these limitations can be resolved pretty easily, but requires some
> design decisions. I can prepare appropriate patches.
> 
> Ad. 1.
> 
> Why are operations on attributes other than than user.* and
> system.posix_acl_* forbidden? Is this due to security reasons?
> 
> If so, can we map all of them to user.virtfs namespace, similarly as
> system.posix_acl_* are being mapped to user.virtfs.system.posix_acl_* in
> 'mapping' mode already? This way any trusted/security/system attributes will
> be effective only when mounted via virtfs inside VM.
> 
> Ad. 2.
> 
> local_readdir() can fill entry->d_type with the right DT_* value by
> obtaining file type from mapping and translating it with IFTODT() macro.
> This would, however, require reading 'user.virtfs.mode' for each direntry
> during readdir() call, what can affect performance. If so, this behavior
> would probably need to be controlled with some runtime option.
> 
> 'mapped-xattr' and 'mapped-file' models are essential for running qemu with
> overlayfs as non-root, because overlayfs creates device nodes, what is
> possible for unprivileged user only with these models.