Message-ID: <CAEivzxdrVWB_jj58bU7kbNBD6kGhDJu_WZSocy4q=-RZEr-wfw@mail.gmail.com>
Date:   Thu, 27 Jul 2023 17:00:28 +0200
From:   Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
To:     Stéphane Graber <stgraber@...ntu.com>
Cc:     Christian Brauner <brauner@...nel.org>,
        Xiubo Li <xiubli@...hat.com>, linux-fsdevel@...r.kernel.org,
        Jeff Layton <jlayton@...nel.org>,
        Ilya Dryomov <idryomov@...il.com>, ceph-devel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 03/11] ceph: handle idmapped mounts in create_request_message()

On Thu, Jul 27, 2023 at 4:46 PM Stéphane Graber <stgraber@...ntu.com> wrote:
>
> On Thu, Jul 27, 2023 at 5:48 AM Aleksandr Mikhalitsyn
> <aleksandr.mikhalitsyn@...onical.com> wrote:
> >
> > On Thu, Jul 27, 2023 at 11:01 AM Christian Brauner <brauner@...nel.org> wrote:
> > >
> > > On Thu, Jul 27, 2023 at 08:36:40AM +0200, Aleksandr Mikhalitsyn wrote:
> > > > On Thu, Jul 27, 2023 at 7:30 AM Xiubo Li <xiubli@...hat.com> wrote:
> > > > >
> > > > >
> > > > > On 7/26/23 22:10, Alexander Mikhalitsyn wrote:
> > > > > > Inode operations that create a new filesystem object such as ->mknod,
> > > > > > ->create, ->mkdir() and others don't take a {g,u}id argument explicitly.
> > > > > > Instead the caller's fs{g,u}id is used for the {g,u}id of the new
> > > > > > filesystem object.
> > > > > >
> > > > > > In order to ensure that the correct {g,u}id is used map the caller's
> > > > > > fs{g,u}id for creation requests. This doesn't require complex changes.
> > > > > > It suffices to pass in the relevant idmapping recorded in the request
> > > > > > message. If this request message was triggered from an inode operation
> > > > > > that creates filesystem objects it will have passed down the relevant
> > > > > > idmaping. If this is a request message that was triggered from an inode
> > > > > > operation that doens't need to take idmappings into account the initial
> > > > > > idmapping is passed down which is an identity mapping.
> > > > > >
> > > > > > This change uses a new cephfs protocol extension CEPHFS_FEATURE_HAS_OWNER_UIDGID
> > > > > > which adds two new fields (owner_{u,g}id) to the request head structure.
> > > > > > So, we need to ensure that MDS supports it otherwise we need to fail
> > > > > > any IO that comes through an idmapped mount because we can't process it
> > > > > > in a proper way. MDS server without such an extension will use caller_{u,g}id
> > > > > > fields to set a new inode owner UID/GID which is incorrect because caller_{u,g}id
> > > > > > values are unmapped. At the same time we can't map these fields with an
> > > > > > idmapping as it can break UID/GID-based permission checks logic on the
> > > > > > MDS side. This problem was described with a lot of details at [1], [2].
> > > > > >
> > > > > > [1] https://lore.kernel.org/lkml/CAEivzxfw1fHO2TFA4dx3u23ZKK6Q+EThfzuibrhA3RKM=ZOYLg@mail.gmail.com/
> > > > > > [2] https://lore.kernel.org/all/20220104140414.155198-3-brauner@kernel.org/
> > > > > >
> > > > > > Cc: Xiubo Li <xiubli@...hat.com>
> > > > > > Cc: Jeff Layton <jlayton@...nel.org>
> > > > > > Cc: Ilya Dryomov <idryomov@...il.com>
> > > > > > Cc: ceph-devel@...r.kernel.org
> > > > > > Co-Developed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
> > > > > > Signed-off-by: Christian Brauner <christian.brauner@...ntu.com>
> > > > > > Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
> > > > > > ---
> > > > > > v7:
> > > > > >       - reworked to use two new fields for owner UID/GID (https://github.com/ceph/ceph/pull/52575)
> > > > > > ---
> > > > > >   fs/ceph/mds_client.c         | 20 ++++++++++++++++++++
> > > > > >   fs/ceph/mds_client.h         |  5 ++++-
> > > > > >   include/linux/ceph/ceph_fs.h |  4 +++-
> > > > > >   3 files changed, 27 insertions(+), 2 deletions(-)
> > > > > >
> > > > > > diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
> > > > > > index c641ab046e98..ac095a95f3d0 100644
> > > > > > --- a/fs/ceph/mds_client.c
> > > > > > +++ b/fs/ceph/mds_client.c
> > > > > > @@ -2923,6 +2923,7 @@ static struct ceph_msg *create_request_message(struct ceph_mds_session *session,
> > > > > >   {
> > > > > >       int mds = session->s_mds;
> > > > > >       struct ceph_mds_client *mdsc = session->s_mdsc;
> > > > > > +     struct ceph_client *cl = mdsc->fsc->client;
> > > > > >       struct ceph_msg *msg;
> > > > > >       struct ceph_mds_request_head_legacy *lhead;
> > > > > >       const char *path1 = NULL;
> > > > > > @@ -3028,6 +3029,16 @@ static struct ceph_msg *create_request_message(struct ceph_mds_session *session,
> > > > > >       lhead = find_legacy_request_head(msg->front.iov_base,
> > > > > >                                        session->s_con.peer_features);
> > > > > >
> > > > > > +     if ((req->r_mnt_idmap != &nop_mnt_idmap) &&
> > > > > > +         !test_bit(CEPHFS_FEATURE_HAS_OWNER_UIDGID, &session->s_features)) {
> > > > > > +             pr_err_ratelimited_client(cl,
> > > > > > +                     "idmapped mount is used and CEPHFS_FEATURE_HAS_OWNER_UIDGID"
> > > > > > +                     " is not supported by MDS. Fail request with -EIO.\n");
> > > > > > +
> > > > > > +             ret = -EIO;
> > > > > > +             goto out_err;
> > > > > > +     }
> > > > > > +
> > > > >
> > > > > I think this couldn't fail the mounting operation, right ?
> > > >
> > > > This won't fail mounting. First of all an idmapped mount is always an
> > > > additional mount, you always
> > > > start from doing "normal" mount and only after that you can use this
> > > > mount to create an idmapped one.
> > > > ( example: https://github.com/brauner/mount-idmapped/tree/master )
> > > >
> > > > >
> > > > > IMO we should fail the mounting from the beginning.
> > > >
> > > > Unfortunately, we can't fail mount from the beginning. Procedure of
> > > > the idmapped mounts
> > > > creation is handled not on the filesystem level, but on the VFS level
> > >
> > > Correct. It's a generic vfsmount feature.
> > >
> > > > (source: https://github.com/torvalds/linux/blob/0a8db05b571ad5b8d5c8774a004c0424260a90bd/fs/namespace.c#L4277
> > > > )
> > > >
> > > > Kernel perform all required checks as:
> > > > - filesystem type has declared to support idmappings
> > > > (fs_type->fs_flags & FS_ALLOW_IDMAP)
> > > > - user who creates idmapped mount should be CAP_SYS_ADMIN in a user
> > > > namespace that owns superblock of the filesystem
> > > > (for cephfs it's always init_user_ns => user should be root on the host)
> > > >
> > > > So I would like to go this way because of the reasons mentioned above:
> > > > - root user is someone who understands what he does.
> > > > - idmapped mounts are never "first" mounts. They are always created
> > > > after "normal" mount.
> > > > - effectively this check makes "normal" mount to work normally and
> > > > fail only requests that comes through an idmapped mounts
> > > > with reasonable error message. Obviously, all read operations will
> > > > work perfectly well only the operations that create new inodes will
> > > > fail.
> > > > Btw, we already have an analogical semantic on the VFS level for users
> > > > who have no UID/GID mapping to the host. Filesystem requests for
> > > > such users will fail with -EOVERFLOW. Here we have something close.
> > >
> > > Refusing requests coming from an idmapped mount if the server misses
> > > appropriate features is good enough as a first step imho. And yes, we do
> > > have similar logic on the vfs level for unmapped uid/gid.
> >
> > Thanks, Christian!
> >
> > I wanted to add that alternative here is to modify caller_{u,g}id
> > fields as it was done in the first approach,
> > it will break the UID/GID-based permissions model for old MDS versions
> > (we can put printk_once to inform user about this),
> > but at the same time it will allow us to support idmapped mounts in
> > all cases. This support will be not fully ideal for old MDS
> >  and perfectly well for new MDS versions.
> >
> > Alternatively, we can introduce cephfs mount option like
> > "idmap_with_old_mds" and if it's enabled then we set caller_{u,g}id
> > for MDS without CEPHFS_FEATURE_HAS_OWNER_UIDGID, if it's disabled
> > (default) we fail requests with -EIO. For
> > new MDS everything goes in the right way.
> >
> > Kind regards,
> > Alex
>
> Hey there,
>
> A very strong +1 on there needing to be some way to make this work
> with older Ceph releases.
> Ceph Reef isn't out yet and we're in July 2023, so I'd really like not
> having to wait until Ceph Squid in mid 2024 to be able to make use of
> this!
>
> Some kind of mount option, module option or the like would all be fine for this.

I really like this way. I can implement it really quickly. Let's just
agree on this :)
It looks like an ideal solution for everyone.

Kind regards,
Alex

>
> Stéphane

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