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

[Namjae Jeon <linkinjeon@kernel.org>]

2021-08-21 20:11 GMT+09:00, 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.
Okay, Thanks for your work!
>
> Christian
>