Message-ID: <fc6ea2e2-3f61-d91d-2336-a73907f5311f@redhat.com>
Date:   Fri, 2 Jun 2023 08:41:27 +0800
From:   Xiubo Li <xiubli@...hat.com>
To:     Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
Cc:     brauner@...nel.org, stgraber@...ntu.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 v2 03/13] ceph: handle idmapped mounts in
 create_request_message()


On 6/2/23 02:29, Aleksandr Mikhalitsyn wrote:
> On Thu, Jun 1, 2023 at 4:29 AM Xiubo Li <xiubli@...hat.com> wrote:
>>
>> On 6/1/23 00:32, Aleksandr Mikhalitsyn wrote:
>>> On Mon, May 29, 2023 at 5:52 AM Xiubo Li <xiubli@...hat.com> wrote:
>>>> On 5/24/23 23:33, Alexander Mikhalitsyn wrote:
>>>>> From: Christian Brauner <christian.brauner@...ntu.com>
>>>>>
>>>>> 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.
>>>>>
>>>>> Cephfs mds creation request argument structures mirror this filesystem
>>>>> behavior. They don't encode a {g,u}id explicitly. Instead the caller's
>>>>> fs{g,u}id that is always sent as part of any mds request is used by the
>>>>> servers to set 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 and thus is
>>>>> guaranteed to leave the caller's fs{g,u}id unchanged.,u}id is sent.
>>>>>
>>>>> The last few weeks before Christmas 2021 I have spent time not just
>>>>> reading and poking the cephfs kernel code but also took a look at the
>>>>> ceph mds server userspace to ensure I didn't miss some subtlety.
>>>>>
>>>>> This made me aware of one complication to solve. All requests send the
>>>>> caller's fs{g,u}id over the wire. The caller's fs{g,u}id matters for the
>>>>> server in exactly two cases:
>>>>>
>>>>> 1. to set the ownership for creation requests
>>>>> 2. to determine whether this client is allowed access on this server
>>>>>
>>>>> Case 1. we already covered and explained. Case 2. is only relevant for
>>>>> servers where an explicit uid access restriction has been set. That is
>>>>> to say the mds server restricts access to requests coming from a
>>>>> specific uid. Servers without uid restrictions will grant access to
>>>>> requests from any uid by setting MDS_AUTH_UID_ANY.
>>>>>
>>>>> Case 2. introduces the complication because the caller's fs{g,u}id is
>>>>> not just used to record ownership but also serves as the {g,u}id used
>>>>> when checking access to the server.
>>>>>
>>>>> Consider a user mounting a cephfs client and creating an idmapped mount
>>>>> from it that maps files owned by uid 1000 to be owned uid 0:
>>>>>
>>>>> mount -t cephfs -o [...] /unmapped
>>>>> mount-idmapped --map-mount 1000:0:1 /idmapped
>>>>>
>>>>> That is to say if the mounted cephfs filesystem contains a file "file1"
>>>>> which is owned by uid 1000:
>>>>>
>>>>> - looking at it via /unmapped/file1 will report it as owned by uid 1000
>>>>>      (One can think of this as the on-disk value.)
>>>>> - looking at it via /idmapped/file1 will report it as owned by uid 0
>>>>>
>>>>> Now, consider creating new files via the idmapped mount at /idmapped.
>>>>> When a caller with fs{g,u}id 1000 creates a file "file2" by going
>>>>> through the idmapped mount mounted at /idmapped it will create a file
>>>>> that is owned by uid 1000 on-disk, i.e.:
>>>>>
>>>>> - looking at it via /unmapped/file2 will report it as owned by uid 1000
>>>>> - looking at it via /idmapped/file2 will report it as owned by uid 0
>>>>>
>>>>> Now consider an mds server that has a uid access restriction set and
>>>>> only grants access to requests from uid 0.
>>>>>
>>>>> If the client sends a creation request for a file e.g. /idmapped/file2
>>>>> it will send the caller's fs{g,u}id idmapped according to the idmapped
>>>>> mount. So if the caller has fs{g,u}id 1000 it will be mapped to {g,u}id
>>>>> 0 in the idmapped mount and will be sent over the wire allowing the
>>>>> caller access to the mds server.
>>>>>
>>>>> However, if the caller is not issuing a creation request the caller's
>>>>> fs{g,u}id will be send without the mount's idmapping applied. So if the
>>>>> caller that just successfully created a new file on the restricted mds
>>>>> server sends a request as fs{g,u}id 1000 access will be refused. This
>>>>> however is inconsistent.
>>>>>
>>>>>    From my perspective the root of the problem lies in the fact that
>>>>> creation requests implicitly infer the ownership from the {g,u}id that
>>>>> gets sent along with every mds request.
>>>>>
>>>>> I have thought of multiple ways of addressing this problem but the one I
>>>>> prefer is to give all mds requests that create a filesystem object a
>>>>> proper, separate {g,u}id field entry in the argument struct. This is,
>>>>> for example how ->setattr mds requests work.
>>>>>
>>>>> This way the caller's fs{g,u}id can be used consistenly for server
>>>>> access checks and is separated from the ownership for new filesystem
>>>>> objects.
>>>>>
>>>>> Servers could then be updated to refuse creation requests whenever the
>>>>> {g,u}id used for access checking doesn't match the {g,u}id used for
>>>>> creating the filesystem object just as is done for setattr requests on a
>>>>> uid restricted server. But I am, of course, open to other suggestions.
>>>>>
>>>>> Cc: Jeff Layton <jlayton@...nel.org>
>>>>> Cc: Ilya Dryomov <idryomov@...il.com>
>>>>> Cc: ceph-devel@...r.kernel.org
>>>>> Signed-off-by: Christian Brauner <christian.brauner@...ntu.com>
>>>>> Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
>>>>> ---
>>>>>     fs/ceph/mds_client.c | 22 ++++++++++++++++++----
>>>>>     1 file changed, 18 insertions(+), 4 deletions(-)
>>>>>
>>>>> diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
>>>>> index 810c3db2e369..e4265843b838 100644
>>>>> --- a/fs/ceph/mds_client.c
>>>>> +++ b/fs/ceph/mds_client.c
>>>>> @@ -2583,6 +2583,8 @@ static struct ceph_msg *create_request_message(struct ceph_mds_session *session,
>>>>>         void *p, *end;
>>>>>         int ret;
>>>>>         bool legacy = !(session->s_con.peer_features & CEPH_FEATURE_FS_BTIME);
>>>>> +     kuid_t caller_fsuid;
>>>>> +     kgid_t caller_fsgid;
>>>>>
>>>>>         ret = set_request_path_attr(req->r_inode, req->r_dentry,
>>>>>                               req->r_parent, req->r_path1, req->r_ino1.ino,
>>>>> @@ -2651,10 +2653,22 @@ static struct ceph_msg *create_request_message(struct ceph_mds_session *session,
>>>>>
>>>>>         head->mdsmap_epoch = cpu_to_le32(mdsc->mdsmap->m_epoch);
>>>>>         head->op = cpu_to_le32(req->r_op);
>>>>> -     head->caller_uid = cpu_to_le32(from_kuid(&init_user_ns,
>>>>> -                                              req->r_cred->fsuid));
>>>>> -     head->caller_gid = cpu_to_le32(from_kgid(&init_user_ns,
>>>>> -                                              req->r_cred->fsgid));
>>>>> +     /*
>>>>> +      * Inode operations that create filesystem objects based on the
>>>>> +      * caller's fs{g,u}id like ->mknod(), ->create(), ->mkdir() etc. don't
>>>>> +      * have separate {g,u}id fields in their respective structs in the
>>>>> +      * ceph_mds_request_args union. Instead the caller_{g,u}id field is
>>>>> +      * used to set ownership of the newly created inode by the mds server.
>>>>> +      * For these inode operations we need to send the mapped fs{g,u}id over
>>>>> +      * the wire. For other cases we simple set req->r_mnt_idmap to the
>>>>> +      * initial idmapping meaning the unmapped fs{g,u}id is sent.
>>>>> +      */
>>>>> +     caller_fsuid = from_vfsuid(req->r_mnt_idmap, &init_user_ns,
>>>>> +                                     VFSUIDT_INIT(req->r_cred->fsuid));
>>>>> +     caller_fsgid = from_vfsgid(req->r_mnt_idmap, &init_user_ns,
>>>>> +                                     VFSGIDT_INIT(req->r_cred->fsgid));
>>>>> +     head->caller_uid = cpu_to_le32(from_kuid(&init_user_ns, caller_fsuid));
>>>>> +     head->caller_gid = cpu_to_le32(from_kgid(&init_user_ns, caller_fsgid));
>>>> Hi Alexander,
>>> Dear Xiubo,
>>>
>>> Thanks for paying attention to this series!
>>>
>>>> You didn't answer Jeff and Greg's concerns in the first version
>>>> https://www.spinics.net/lists/ceph-devel/msg53356.html.
>>> I've tried to respin discussion in the -v1 thread:
>>> https://lore.kernel.org/all/20230519134420.2d04e5f70aad15679ab566fc@canonical.com/
>>>
>>> No one replied, so I decided to send rebased and slightly changed -v2,
>>> where I've fixed this:
>>> https://lore.kernel.org/all/041afbfd171915d62ab9a93c7a35d9c9d5c5bf7b.camel@kernel.org/
>>>
>>>> I am also confused as Greg mentioned. If we just map the ids as 1000:0
>>>> and created a file and then map the ids 1000:10, then the file couldn't
>>>> be accessible, right ? Is this normal and as expected ?
>>> This can be a problem only if filtering based on the UID is turned on
>>> on the server side (which is a relatively rare case).
>>>
>>> idmapped mounts are not about mapping a caller UID/GID, idmapped
>>> mounts are about mapping inode owner's UID/GID.
>>> So, for example if you have UID 1000 (on disk) and have an idmapping
>>> 1000:0 then it will be shown as owned by 0.
>> My understanding was that on the disk the files' owner UID should be
>> 1000 always, while in the client side it will show file's owner as the
>> mapped UID 0 with an idmapping 1000:0.
> Hi, Xiubo!
>
>> This should be the same as what you mentioned above, right ?
> Right.
>
> Let me show a real output from a real command line experiment :-)
>
> 1. Mount cephfs
>
> mount.ceph admin@....cephfs=/ /mnt/ceph -o
> mon_addr=127.0.0.1:6789,secret=very_secret_key
>
> 2. Make 1000:1000 a root dentry owner (it will be convenient because
> we want to use mapping 1000:0:1 for simplicity)
>
> chown 1000:1000 /mnt/ceph
>
> 3. create an idmapped mount based on a regular /mnt/ceph mount using a
> mount-idmapped tool that was written by Christian.
> [ taken from https://raw.githubusercontent.com/brauner/mount-idmapped/master/mount-idmapped.c
> ]
>
> ./mount-idmapped --map-mount b:1000:0:1 /mnt/ceph /mnt/ceph_idmapped
>
> "b" stands for "both", so we are creating a mapping of length 1 for
> both UID and GID.
> 1000 is a UID/GID "on-disk", 0 is a mapped UID/GID.
>
> 4. Just to be precise, let's look at which UID/GID we have now.
>
> root@...ntu:/home/ubuntu# ls -lan /mnt/ceph
> total 4
> drwxrwxrwx 2 1000 1000    0 Jun  1 17:51 .
> drwxr-xr-x 4    0    0 4096 Jun  1 16:55 ..
>
> root@...ntu:/home/ubuntu# ls -lan /mnt/ceph_idmapped
> total 4
> drwxrwxrwx 2 0 0    0 Jun  1 17:51 .
> drwxr-xr-x 4 0 0 4096 Jun  1 16:55 ..
>
> 5. Now let's create a bunch of files with different owners and through
> different mounts (idmapped/non-idmapped).
>
> 5.1. Create a file from 0:0 through the idmapped mount (it should
> appear as 1000:1000 on disk)
> root@...ntu:/home/ubuntu# sudo -u#0 -g#0 touch
> /mnt/ceph_idmapped/created_through_idmapped_mnt_with_uid0
>
> 5.2. Create a file from 1000:1000 through the idmapped mount (should
> fail because 1000:1000 is not a valid UID/GID as it can't be mapped
> back to the "on-disk" UID/GID set).
> root@...ntu:/home/ubuntu# sudo -u#1000 -g#1000 touch
> /mnt/ceph_idmapped/created_through_idmapped_mnt_with_uid1000
> touch: cannot touch
> '/mnt/ceph_idmapped/created_through_idmapped_mnt_with_uid1000': Value
> too large for defined data type
>
> ... and we've got EOVERFLOW. That's correct!
>
> 5.3. Create a file from 0:0 but through the regular mount. (it should
> appear as overflowuid(=65534) in idmapped mount, because 0:0 on-disk
> is not mapped to the UID/GID set).
>
> root@...ntu:/home/ubuntu# sudo -u#0 -g#0 touch
> /mnt/ceph/created_directly_with_uid0
>
> 5.4. Create a file from 1000:1000 but through the regular mount. (it
> should appear as 0:0 in idmapped mount, because 1000 (on-disk) mapped
> to 0).
>
> root@...ntu:/home/ubuntu# sudo -u#1000 -g#1000 touch
> /mnt/ceph/created_directly_with_uid1000
>
> 6. Now let's look on the result:
>
> root@...ntu:/home/ubuntu# ls -lan /mnt/ceph
> total 4
> drwxrwxrwx 2 1000 1000    3 Jun  1 17:54 .
> drwxr-xr-x 4    0    0 4096 Jun  1 16:55 ..
> -rw-r--r-- 1    0    0    0 Jun  1 17:54 created_directly_with_uid0
> -rw-rw-r-- 1 1000 1000    0 Jun  1 17:54 created_directly_with_uid1000
> -rw-r--r-- 1 1000 1000    0 Jun  1 17:53 created_through_idmapped_mnt_with_uid0
>
> root@...ntu:/home/ubuntu# ls -lan /mnt/ceph_idmapped
> total 4
> drwxrwxrwx 2     0     0    3 Jun  1 17:54 .
> drwxr-xr-x 4     0     0 4096 Jun  1 16:55 ..
> -rw-r--r-- 1 65534 65534    0 Jun  1 17:54 created_directly_with_uid0
> -rw-rw-r-- 1     0     0    0 Jun  1 17:54 created_directly_with_uid1000
> -rw-r--r-- 1     0     0    0 Jun  1 17:53
> created_through_idmapped_mnt_with_uid0
>
>>> If you create a file from a user with UID 0 then you will get UID 1000
>>> on disk. To achieve that, we map a current user fs{g,u}id
>>> when sending a creation request according to the idmapping mount to
>>> make things consistent.
>> As you know the cephfs MDSs will use the creation requests' caller UID
>> as the owner's UID when creating new inodes.
> Yes, that's why we have to map a caller UID to end up with the correct
> value of a file owner.
>
Hmm, I think my understanding was incorrect. This patch here is trying 
to get the correct value of UID 1000 from a mapped mount, which the UID 0.


>> Which means that if the creation requests switches to use the mapped UID
>> 0 as the caller UID then the file's owner will be UID 0 instead of UID
>> 1000 in cephfs MDSs. Does this what this patch want to do ?
> In my example we have a caller with UID equal 0, then the mapped UID
> will be 1000. So, the file will be created with UID = 1000.

Okay, thanks for your above example it helped me to understand the idmap 
logic. Before I tried to read the xfstests test cases and VFS code about 
the idmap but didn't totally done yet.

I will test and review the patches again today or next week.

Thanks

- Xiubo

>>
>>>    But when a user opens a file,
>>> we are sending UID/GID as they are without applying an idmapping.
>> If my understanding is correct above, then when opening the file with
>> non-mapped UID 1000 it may fail because the files' owner is UID 0.
>>
>> Correct me if my understanding is wrong.
>>
>>>    Of
>>> course, generic_permission() kernel helper is aware of
>>> mount idmapping
>> Yeah, this was also what I thought it should be.
>>
>> There is another client auth feature [1] for cephfs. The MDS will allow
>> us to set a path restriction for specify UID, more detail please see [2]:
>>
>>    allow rw path=/dir1 uid=1000 gids=1000
>>
>> This may cause the creation requests to fail if you set the caller UID
>> to the mapped UID.
> Yes, that can be a problem of course. But it will only affect users
> who want to use this feature and it doesn't open any security holes.
> It's just a limitation of this approach. Unfortunately it's barely
> fixable without massive VFS changes and until we have no real use
> cases
> for this combination of idmapped mounts + MDS UID/GID-based path
> restriction we are not sure that it makes sense to implement this
> right now.
>
>>
>> [1] https://docs.ceph.com/en/latest/cephfs/client-auth/
>> [2] https://tracker.ceph.com/issues/59388
> Thanks, I'll take a look closer at that!
>
> Thanks for closely looking into this patchset, Xiubo!
>
> Kind regards,
> Alex
>
>>
>> Thanks
>>
>> - Xiubo
>>
>>> and before open request will go to the server we will
>>> check that current user is allowed to open this file (and during
>>> this check UID/GID of a current user and UID/GID of the file owner
>>> will be properly compared). I.e. this issue is only relevant for the
>>> case
>>> when we have additional permission checks on the network file system
>>> server side.
>>>
>>>> IMO the idmapping should be client-side feature and we should make it
>>>> consistent by using the unmapped fs{g,u}id always here.
>>> To make the current user fs{g,u}id always idmapped we need to make
>>> really big changes in the VFS layer. And it's not obvious
>>> that it justifies the cost. Because this particular feature with
>>> Cephfs idmapped mounts is already used/tested with LXD/LXC workloads
>>> and it works perfectly well. And as far as I know, LXD/LXC were the
>>> first idmapped mount adopters. IMHO, it's better to
>>> start from this approach and if someone will want to extend this
>>> functionality for network filesystems and want to map fs{g,u}id which
>>> are sent over the
>>> wire we will take a look at that. Because anyway, integration with
>>> Cephfs is important for the LXD project and we are looking closely at
>>> this.
>>>
>>> Kind regards,
>>> Alex
>>>
>>>> Thanks
>>>>
>>>> - Xiubo
>>>>
>>>>>         head->ino = cpu_to_le64(req->r_deleg_ino);
>>>>>         head->args = req->r_args;
>>>>>

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