RE: [PATCH] ksmbd: fix lookup on idmapped mounts

["Namjae Jeon" <namjae.jeon@samsung.com>]

> On Tue, Aug 17, 2021 at 08:30:55AM +0900, Namjae Jeon wrote:
> > > From: Christian Brauner <christian.brauner@ubuntu.com>
> > >
> > > It's great that the new in-kernel ksmbd server will support idmapped
> > > mounts out of the box! However, lookup is currently broken. Lookup
> > > helpers such as lookup_one_len() call inode_permission() internally
> > > to ensure that the caller is privileged over the inode of the base dentry they are trying to
> lookup under. So the permission checking here is currently wrong.
> > >
> > > Linux v5.15 will gain a new lookup helper lookup_one() that does
> > > take idmappings into account. I've added it as part of my patch
> > > series to make btrfs support idmapped mounts. The new helper is in
> > > linux- next as part of David's (Sterba) btrfs for-next branch as commit c972214c133b ("namei: add
> mapping aware lookup helper").
> > >
> > > I've said it before during one of my first reviews: I would very much recommend adding fstests to
> [1].
> > > It already seems to have very rudimentary cifs support. There is a
> > > completely generic idmapped mount testsuite that supports idmapped mounts.
> > >
> > > [1]: https://git.kernel.org/pub/scm/fs/xfs/xfsprogs-dev.git/
> > > Cc: Steve French <stfrench@microsoft.com>
> > > Cc: Christoph Hellwig <hch@infradead.org>
> > > Cc: Namjae Jeon <namjae.jeon@samsung.com>
> > > Cc: Hyunchul Lee <hyc.lee@gmail.com>
> > > Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
> > > Cc: David Sterba <dsterba@suse.com>
> > > Cc: linux-cifs@vger.kernel.org
> > > Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
> > > ---
> > Hi Christian,
> >
> > > I merged David's for-next tree into cifsd-next to test this. I did
> > > only compile test this. If someone gives me a clear set of
> > > instructions how to test ksmbd on my local machine I can at least
> > > try to cut some time out of my week to do more reviews. (I'd
> > > especially like to see acl behavior with ksmbd.)
> >
> > There is "How to run ksmbd" section in patch cover letter.
> >
> > https://protect2.fireeye.com/v1/url?k=65ecaaf0-3a779239-65ed21bf-0cc47
> > a336fae-53bc47005a1a97a9&q=1&e=e44c9f9f-d7ae-4768-8cc2-8f02d748fc6e&u=
> > https%3A%2F%2Flkml.org%2Flkml%2F2021%2F8%2F5%2F54
> >
> > Let me know if it doesn't work well even if you try to run it with this step.
> > And We will also check whether your patch work fine.
> >
> > >
> > > One more thing, the tree for ksmbd was very hard to find. I had to do a lot archeology to end up
> at:
> > >
> > > git://git.samba.org/ksmbd.git
> > This is also in the patch cover letter. See "Mailing list and repositories" section.
> > I think that you can use :
> >
> > https://protect2.fireeye.com/v1/url?k=8af83a5d-d5630294-8af9b112-0cc47
> > a336fae-e471ffbdb93d05b7&q=1&e=e44c9f9f-d7ae-4768-8cc2-8f02d748fc6e&u=
> > https%3A%2F%2Fgithub.com%2Fnamjaejeon%2Fsmb3-kernel%2Ftree%2Fksmbd-v7-
> > series
> >
> > >
> > > Would be appreciated if this tree could be reflected in MAINTAINERS
> > > or somewhere else. The github repos with the broken out patches/module aren't really that helpful.
> > Okay, I will add git address of ksmbd in MAINTAINERS on next spin.
> >
> > >
> > > Thanks!
> > > Christian
> > Really thanks for your review and I will apply this patch after checking it.
> 
> Thank your for the pointers.
> 
> Ok, so I've been taking the time to look into cifs and ksmbd today. My mental model was wrong. There
> are two things to consider here:
> 
> 1. server: idmapped mounts with ksmbd
> 2. client: idmapped mounts with cifs
> 
> Your patchset adds support for 1.
Right.

> Let's say I have the following ksmbd config:
> 
> root@f2-vm:~# cat /etc/ksmbd/smb.conf
> [global]
>         netbios name = SMBD
>         server max protocol = SMB3
> [test]
>         path = /opt
>         writeable = yes
>         comment = TEST
>         read only = no
> 
> So /opt can be an idmapped mount and ksmb would know how to deal with that correctly, i.e. you could
> do:
> 
> mount-idmapped --map-mount=b:1000:0:1 /opt /opt
> 
> ksmbd.mountd
> 
> and ksmbd would take the idmapping of /opt into account.
Right.

> 
> That however is different from 2. which is cifs itself being idmappable.
Right.

> Whether or not that makes sense or is needed will need some thinking.
> 
> In any case, this has consequences for xfstests and now I understand your earlier confusion. In
> another mail you pointed out that cifs reports that idmapped mounts are not supported. That is correct
> insofar as it means 2. is not supported, i.e. you can't do:
Right.

> 
> mount -t cifs -o username=foo,password=bar //server/files /mnt
> 
> and then
> 
> mount-idmapped --map-mount=b:1000:0:1 /mnt /mnt
> 
> but that's also not what you want in order to test for ksmbd. What you want is to test 1.
Right. So we have manually tested it, not xfstests.

> 
> So your test setup would require you to setup an idmapped mount and have ksmbd use that which can then
> be mounted by a client.
> 
> With your instructions I was at least able to get a ksmb instance running and be able to mount a
> client with -t cifs. All on the same machine, i.e. my server is localhost.
Okay.

> 
> However, I need to dig a bit into the semantics to make better assertions about what's going on.
Okay. And I have applied your patch to ksmbd.

> 
> Are unix extension supported with ksmb? Everytime I try to use "posix"
> as a mount option with mount -t cifs -o //127.0.0.1/test /mnt I get "uid=0" and "gid=0" and "noposix".
> I do set "unix extensions = yes" in both the samba and ksmbd smb.conf.
With posix mount option, It should work. It worked well before but it is strange now.

I'm not sure this is the correct fix, But could you please try to mount with the below change ?

diff --git a/fs/cifs/fs_context.c b/fs/cifs/fs_context.c
index eed59bc1d913..5fd0b0ddcc57 100644
--- a/fs/cifs/fs_context.c
+++ b/fs/cifs/fs_context.c
@@ -1268,8 +1268,10 @@ static int smb3_fs_context_parse_param(struct fs_context *fc,
        case Opt_unix:
                if (result.negated)
                        ctx->linux_ext = 0;
-               else
+               else {
+                       ctx->linux_ext = 1;
                        ctx->no_linux_ext = 1;
+               }
                break;
        case Opt_nocase:
                ctx->nocase = 1;

Thanks for your work!
> 
> Christian