From: Sargun Dhillon <sargun@sargun.me>
To: "J . Bruce Fields" <bfields@fieldses.org>,
Chuck Lever <chuck.lever@oracle.com>,
Trond Myklebust <trond.myklebust@hammerspace.com>,
Anna Schumaker <anna.schumaker@netapp.com>,
David Howells <dhowells@redhat.com>
Cc: linux-nfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [RESEND PATCH v2 0/3] NFS User Namespaces with new mount API
Date: Sat, 17 Oct 2020 23:59:34 +0000 [thread overview]
Message-ID: <20201017235933.GA1516@ircssh-2.c.rugged-nimbus-611.internal> (raw)
In-Reply-To: <20201016124550.10739-1-sargun@sargun.me>
On Fri, Oct 16, 2020 at 05:45:47AM -0700, Sargun Dhillon wrote:
> This patchset adds some functionality to allow NFS to be used from
> NFS namespaces (containers).
>
> Changes since v1:
> * Added samples
>
> Sargun Dhillon (3):
> NFS: Use cred from fscontext during fsmount
> samples/vfs: Split out common code for new syscall APIs
> samples/vfs: Add example leveraging NFS with new APIs and user
> namespaces
>
> fs/nfs/client.c | 2 +-
> fs/nfs/flexfilelayout/flexfilelayout.c | 1 +
> fs/nfs/nfs4client.c | 2 +-
> samples/vfs/.gitignore | 2 +
> samples/vfs/Makefile | 5 +-
> samples/vfs/test-fsmount.c | 86 +-----------
> samples/vfs/test-nfs-userns.c | 181 +++++++++++++++++++++++++
> samples/vfs/vfs-helper.c | 43 ++++++
> samples/vfs/vfs-helper.h | 55 ++++++++
> 9 files changed, 289 insertions(+), 88 deletions(-)
> create mode 100644 samples/vfs/test-nfs-userns.c
> create mode 100644 samples/vfs/vfs-helper.c
> create mode 100644 samples/vfs/vfs-helper.h
>
> --
> 2.25.1
>
Digging deeper into this a little bit, I actually found that there is some
problematic aspects of the current behaviour. Because nfs_get_tree_common calls
sget_fc, and sget_fc sets the super block's s_user_ns (via alloc_super) to the
fs_context's user namespace unless the global flag is set (which NFS does not
set), there are a bunch of permissions checks that are done against the super
block's user_ns.
It looks like this was introduced in:
f2aedb713c28: NFS: Add fs_context support[1]
It turns out that unmapped users in the "parent" user namespace just get an
EOVERFLOW error when trying to perform a read, even if the UID sent to the NFS
server to read a file is a valid uid (the uid in the init user ns), and
inode_permission checks permissions against the mapped UID in the namespace,
while the authentication credentials (UIDs, GIDs) sent to the server are
those from the init user ns.
[This is all under the assumption there's not upcalls doing ID mapping]
Although, I do not think this presents any security risk (because you have to
have CAP_SYS_ADMIN in the init user ns to get this far), it definitely seems
like "incorrect" behaviour.
[1]: https://lore.kernel.org/linux-nfs/20191120152750.6880-26-smayhew@redhat.com/
prev parent reply other threads:[~2020-10-18 0:00 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-16 12:45 [RESEND PATCH v2 0/3] NFS User Namespaces with new mount API Sargun Dhillon
2020-10-16 12:45 ` [RESEND PATCH v2 1/3] NFS: Use cred from fscontext during fsmount Sargun Dhillon
2020-10-16 12:45 ` [RESEND PATCH v2 2/3] samples/vfs: Split out common code for new syscall APIs Sargun Dhillon
2020-10-16 12:45 ` [RESEND PATCH v2 3/3] samples/vfs: Add example leveraging NFS with new APIs and user namespaces Sargun Dhillon
2020-10-17 23:59 ` Sargun Dhillon [this message]
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