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

[Christian Brauner <christian.brauner@ubuntu.com>]

On Sat, Aug 21, 2021 at 02:59:21PM +0900, Namjae Jeon wrote:
> 2021-08-19 22:01 GMT+09:00, Christian Brauner <christian.brauner@ubuntu.com>:
> > On Thu, Aug 19, 2021 at 11:19:04AM +0900, Namjae Jeon wrote:
> >> > 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;
> >
> > That stops the bleeding indeed. :)
> Okay, sorry for late response.
> > I think a slightly nicer fix might be:
> >
> > diff --git a/fs/cifs/fs_context.c b/fs/cifs/fs_context.c
> > index eed59bc1d913..424b8dc2232e 100644
> > --- a/fs/cifs/fs_context.c
> > +++ b/fs/cifs/fs_context.c
> > @@ -1269,7 +1269,8 @@ static int smb3_fs_context_parse_param(struct
> > fs_context *fc,
> >                 if (result.negated)
> >                         ctx->linux_ext = 0;
> >                 else
> > -                       ctx->no_linux_ext = 1;
> > +                       ctx->linux_ext = 1;
> > +               ctx->no_linux_ext = !ctx->linux_ext;
> >                 break;
> >         case Opt_nocase:
> >                 ctx->nocase = 1;
> >
> > So with this patch applied I got unix permissions working after all. So
> > I was able to do some more testing.
> Okay.
> >
> > Just a few questions (independent of idmapped mounts):
> >
> > 1. Are the "uid=" and "gid=" mount options ignored when "username=" is
> >    specified and "posix" is specified?
> >
> >    It seems that "uid=" and "gid=" have are silently ignored when
> >    "posix' is set. They are neither used to report file ownership under
> >    the cifs mountpoint nor are they relevant when determining ownership
> >    on disk?
> >
> >    As an example, assume I have added a user "foo" with uid 1000 to
> >    ksmbd via:
> >
> >            ksmbd.adduser -a foo
> >
> >    And I mounted a share via:
> >
> >            mount -t cifs -o
> > username=foo,password=bar,posix,uid=1234,gid=1234,forceuid,forcegid
> > //127.0.0.1/test /mnt
> >
> >    i) Ownership in /mnt appears posix-correct, i.e. "uid=" and "gid=" have
> >       no effect on the reported ownership.
> >
> >    ii) Assume I'm logged in as the root user with uid 0. If I create
> >        file or directory in /mnt it will be owned by user foo, i.e. uid
> >        1000, i.e., the "uid=1234" and "gid=1234" mount option have zero
> >        effect on the ownership of the files?
> >
> > 2. Are the "uid=" and "gid=" options ignored for permission checking
> >    when "posix" is specified?
> >
> > 3. Am I correct in assuming that there is no mount option that makes
> >    chown() or chmod() have an actual effect.
> That will be an answer for 1,2, 3 question. There are mount options for this.
>  1. modefromsid
>  2. idsfromsid
> 
> You can use this mount option and please check it.

Thank you! This works and finally I can hit some codepaths I wasn't able
to until now.

> >
> >    cifs seems to have support for it but the ksmbd server doesn't seem
> >    to?
> No, you need to use mount options for this as I said.
> ksmbd have supported it but I found an issue related to chown and have fixed.
> 
> Could you please check the below git branch (pulled David'tree + ksmbd fixes) ?
> 
>   git clone --branch=for-christian https://github.com/namjaejeon/smb3-kernel

Thanks, I've pulled that branch.

I have a some patches for ksmb that I'll be sending out next week. I
just need to test the changes and verify that it all makes sense.

Christian