Re: [PATCH v5 0/2] NFS: Fix interaction between fs_context and user namespaces

[Sargun Dhillon <sargun@sargun.me>]

On Thu, Nov 12, 2020 at 02:09:50AM -0800, Sargun Dhillon wrote:
> Right now, it is possible to mount NFS with an non-matching super block
> user ns, and NFS sunrpc user ns. This (for the user) results in an awkward
> set of interactions if using anything other than auth_null, where the UIDs
> being sent to the server are different than the local UIDs being checked.
> This can cause "breakage", where if you try to communicate with the NFS
> server with any other set of mappings, it breaks.
> 
> The reason for this is that you can call fsopen("nfs4") in the unprivileged
> namespace, and that configures fs_context with all the right information
> for that user namespace. In addition, it also keeps a gets a cred object
> associated with the caller -- which should match the user namespace.
> Unfortunately, the mount has to be finished in the init_user_ns because we
> currently require CAP_SYS_ADMIN in the init user namespace to call fsmount.
> This means that the superblock's user namespace is set "correctly" to the
> container, but there's absolutely no way nfs4idmap to consume an
> unprivileged user namespace because the cred / user_ns that's passed down
> to nfs4idmap is the one at fsmount.
> 
> How this actually exhibits is let's say that the UID 0 in the user
> namespace is mapped to UID 1000 in the init user ns (and kuid space). What
> will happen is that nfs4idmap will translate the UID 1000 into UID 0 on the
> wire, even if the mount is in entirely in the mount / user namespace of the
> container.
> 
> So, it looks something like this
> Client in unprivileged User NS (UID: 0, KUID: 0)
> 	->Perform open()
> 		...VFS / NFS bits...
> 		nfs_map_uid_to_name ->
> 			from_kuid_munged(init_user_ns, uid) (returns 0)
> 				RPC with UID 0
> 
> This behaviour happens "the other way" as well, where the UID in the
> container may be 0, but the corresponding kuid is 1000. When a response
> from an NFS server comes in we decode it according to the idmap userns.
> The way this exhibits is even more odd.
> 
> Server responds with file attribute (UID: 0, GID: 0)
> 	->nfs_map_name_to_uid(..., 0)
> 		->make_kuid(init_user_ns, id) (returns 0)
> 			....VFS / NFS Bits...
> 			->from_kuid(container_ns, 0) -> invalid uid
> 				-> EOVERFLOW
> 
> This changes the nfs server to use the cred / userns from fs_context, which
> is how idmap is constructed. This subsequently is used in the above
> described flow of converting uids back-and-forth.
> 
> Trond gave the feedback that this behaviour [implemented by this patch] is
> how the legacy sys_mount() behaviour worked[1], and that the intended
> behaviour is for UIDs to be plumbed through entirely, where the user
> namespaces UIDs are what is sent over the wire, and not the init user ns.
> 
> [1]: https://lore.kernel.org/linux-nfs/8feccf45f6575a204da03e796391cc135283eb88.camel@hammerspace.com/
> 
> Sargun Dhillon (2):
>   NFS: NFSv2/NFSv3: Use cred from fs_context during mount
>   NFSv4: Refactor to use user namespaces for nfs4idmap
> 
>  fs/nfs/client.c     | 4 ++--
>  fs/nfs/nfs4client.c | 2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> 
> base-commit: 8c39076c276be0b31982e44654e2c2357473258a
> -- 
> 2.25.1
> 
Trond,

I was just thinking, since you said that this is the behaviour of the sys_mount 
API, would this be considered a regression? Should it go to stable (v5.9)?