Messages in this thread | | | Date | Fri, 2 Jun 2023 08:41:27 +0800 | Subject | Re: [PATCH v2 03/13] ceph: handle idmapped mounts in create_request_message() | From | Xiubo Li <> |
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On 6/2/23 02:29, Aleksandr Mikhalitsyn wrote: > On Thu, Jun 1, 2023 at 4:29 AM Xiubo Li <xiubli@redhat.com> wrote: >> >> On 6/1/23 00:32, Aleksandr Mikhalitsyn wrote: >>> On Mon, May 29, 2023 at 5:52 AM Xiubo Li <xiubli@redhat.com> wrote: >>>> On 5/24/23 23:33, Alexander Mikhalitsyn wrote: >>>>> From: Christian Brauner <christian.brauner@ubuntu.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@kernel.org> >>>>> Cc: Ilya Dryomov <idryomov@gmail.com> >>>>> Cc: ceph-devel@vger.kernel.org >>>>> Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com> >>>>> Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.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@XYZ.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@ubuntu:/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@ubuntu:/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@ubuntu:/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@ubuntu:/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@ubuntu:/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@ubuntu:/home/ubuntu# sudo -u#1000 -g#1000 touch > /mnt/ceph/created_directly_with_uid1000 > > 6. Now let's look on the result: > > root@ubuntu:/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@ubuntu:/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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