On Mon, 7 Nov 2016, Gregory Farnum wrote:
> On Mon, Nov 7, 2016 at 10:39 AM, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> > On Mon, 2016-11-07 at 14:36 +0000, Sage Weil wrote:
> >> On Mon, 7 Nov 2016, Jeff Layton wrote:
> >> > On Mon, 2016-11-07 at 14:05 +0000, Sage Weil wrote:
> >> > > On Mon, 7 Nov 2016, Jeff Layton wrote:
> >> > > > On Mon, 2016-11-07 at 16:43 +0800, Yan, Zheng wrote:
> >> > > > > On Fri, Nov 4, 2016 at 8:57 PM, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> >> > > > > >
> >> > > > > > On Fri, 2016-11-04 at 07:34 -0400, Jeff Layton wrote:
> >> > > > > > >
> >> > > > > > > The userland ceph has MClientCaps at struct version 9. This brings the
> >> > > > > > > kernel up the same version.
> >> > > > > > >
> >> > > > > > > With this change, we have to start tracking the btime and change_attr,
> >> > > > > > > so that the client can pass back sane values in cap messages. The
> >> > > > > > > client doesn't care about the btime at all, so this is just passed
> >> > > > > > > around, but the change_attr is used when ceph is exported via NFS.
> >> > > > > > >
> >> > > > > > > For now, the new "sync" parm is left at 0, to preserve the existing
> >> > > > > > > behavior of the client.
> >> > > > > > >
> >> > > > > > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> >> > > > > > > ---
> >> > > > > > > fs/ceph/caps.c | 33 +++++++++++++++++++++++++--------
> >> > > > > > > 1 file changed, 25 insertions(+), 8 deletions(-)
> >> > > > > > >
> >> > > > > > > diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
> >> > > > > > > index 6e99866b1946..452f5024589f 100644
> >> > > > > > > --- a/fs/ceph/caps.c
> >> > > > > > > +++ b/fs/ceph/caps.c
> >> > > > > > > @@ -991,9 +991,9 @@ struct cap_msg_args {
> >> > > > > > > struct ceph_mds_session *session;
> >> > > > > > > u64 ino, cid, follows;
> >> > > > > > > u64 flush_tid, oldest_flush_tid, size, max_size;
> >> > > > > > > - u64 xattr_version;
> >> > > > > > > + u64 xattr_version, change_attr;
> >> > > > > > > struct ceph_buffer *xattr_buf;
> >> > > > > > > - struct timespec atime, mtime, ctime;
> >> > > > > > > + struct timespec atime, mtime, ctime, btime;
> >> > > > > > > int op, caps, wanted, dirty;
> >> > > > > > > u32 seq, issue_seq, mseq, time_warp_seq;
> >> > > > > > > kuid_t uid;
> >> > > > > > > @@ -1026,13 +1026,13 @@ static int send_cap_msg(struct cap_msg_args *arg)
> >> > > > > > >
> >> > > > > > > /* flock buffer size + inline version + inline data size +
> >> > > > > > > * osd_epoch_barrier + oldest_flush_tid */
> >> > > > > > > - extra_len = 4 + 8 + 4 + 4 + 8;
> >> > > > > > > + extra_len = 4 + 8 + 4 + 4 + 8 + 4 + 4 + 4 + 8 + 8 + 1;
> >> > > > > > > msg = ceph_msg_new(CEPH_MSG_CLIENT_CAPS, sizeof(*fc) + extra_len,
> >> > > > > > > GFP_NOFS, false);
> >> > > > > > > if (!msg)
> >> > > > > > > return -ENOMEM;
> >> > > > > > >
> >> > > > > > > - msg->hdr.version = cpu_to_le16(6);
> >> > > > > > > + msg->hdr.version = cpu_to_le16(9);
> >> > > > > > > msg->hdr.tid = cpu_to_le64(arg->flush_tid);
> >> > > > > > >
> >> > > > > > > fc = msg->front.iov_base;
> >> > > > > > > @@ -1068,17 +1068,30 @@ static int send_cap_msg(struct cap_msg_args *arg)
> >> > > > > > > }
> >> > > > > > >
> >> > > > > > > p = fc + 1;
> >> > > > > > > - /* flock buffer size */
> >> > > > > > > + /* flock buffer size (version 2) */
> >> > > > > > > ceph_encode_32(&p, 0);
> >> > > > > > > - /* inline version */
> >> > > > > > > + /* inline version (version 4) */
> >> > > > > > > ceph_encode_64(&p, arg->inline_data ? 0 : CEPH_INLINE_NONE);
> >> > > > > > > /* inline data size */
> >> > > > > > > ceph_encode_32(&p, 0);
> >> > > > > > > - /* osd_epoch_barrier */
> >> > > > > > > + /* osd_epoch_barrier (version 5) */
> >> > > > > > > ceph_encode_32(&p, 0);
> >> > > > > > > - /* oldest_flush_tid */
> >> > > > > > > + /* oldest_flush_tid (version 6) */
> >> > > > > > > ceph_encode_64(&p, arg->oldest_flush_tid);
> >> > > > > > >
> >> > > > > > > + /* caller_uid/caller_gid (version 7) */
> >> > > > > > > + ceph_encode_32(&p, (u32)-1);
> >> > > > > > > + ceph_encode_32(&p, (u32)-1);
> >> > > > > >
> >> > > > > > A bit of self-review...
> >> > > > > >
> >> > > > > > Not sure if we want to set the above to something else -- maybe 0 or to
> >> > > > > > current's creds? That may not always make sense though (during e.g.
> >> > > > > > writeback).
> >> > > > > >
> >> > > >
> >> > > > Looking further, I'm not quite sure I understand why we send creds at
> >> > > > all in cap messages. Can you clarify where that matters?
> >> > > >
> >> > > > The way I look at it, would be to consider caps to be something like a
> >> > > > more granular NFS delegation or SMB oplock.
> >> > > >
> >> > > > In that light, a cap flush is just the client sending updated attrs for
> >> > > > the exclusive caps that it has already been granted. Is there a
> >> > > > situation where we would ever want to refuse that update?
> >> > >
> >> > > A chmod or chown can be done locally if you have excl caps and flushed
> >> > > back to the MDS via a caps message. We need to verify the user has
> >> > > permission to make the change.
> >> > >
> >> >
> >> > My take is that once the MDS has delegated Ax to the client, then it's
> >> > effectively trusting the client to handle permissions enforcement
> >> > correctly. I don't see why we should second guess that.
> >> >
> >> > > > Note that nothing ever checks the return code for _do_cap_update in the
> >> > > > userland code. If the permissions check fails, then we'll end up
> >> > > > silently dropping the updated attrs on the floor.
> >> > >
> >> > > Yeah. This was mainly for expediency... the protocol assumes that flushes
> >> > > don't fail. Given that the client does it's own permissions check, I
> >> > > think the way to improve this is to have it prevent the flush in the first
> >> > > place, so that it's only nefarious clients that are effected (and who
> >> > > cares if they get confused). I don't think we have a particularly good
> >> > > way to tell the client it can't, say, sudo chmod 0:0 a file, though.
> >> > >
> >> >
> >> > Sorry, I don't quite follow. How would we prevent the flush from a
> >> > nefarious client (which is not something we can really control)?
> >> >
> >> > In any case...ISTM that the permissions check in _do_cap_update ought to
> >> > be replaced by a cephx key check. IOW, what we really want to know is
> >> > whether the client is truly the one to which we delegated the caps. If
> >> > so, then we sort of have to trust that it's doing the right thing with
> >> > respect to permissions checking here.
> >>
> >> The capability can say "you are allowed to be uid 1000 or uid 1020." We
> >> want to delegate the EXCL caps to the client so that a create + chmod +
> >> chown + write can all happen efficiently, but we still need to ensure that
> >> the values they set are legal (a permitted uid/gid combo).
> >>
> >> A common example would be user workstations that are allowed access to
> >> /home/user and restricted via their mds caps to their uid/gid. We need to
> >> prevent them from doing a 'sudo chown 0:0 foo'...
> >>
> >>
> >
> >
> > On what basis do you make such a decision though? For instance, NFS does
> > root-squashing which is (generally) a per-export+per-client thing.
> > It sounds like you're saying that ceph has different semantics here?
> >
> > (cc'ing Greg here)
>
> As Sage says, we definitely avoid the root squash semantics. We
> discussed them last year and concluded they were an inappropriate
> match for Ceph's permission model.
>
> >
> > Also, chown (at least under POSIX) is reserved for superuser only, and
> > now that I look, I think this check in MDSAuthCaps::is_capable may be
> > wrong:
> >
> > // chown/chgrp
> > if (mask & MAY_CHOWN) {
> > if (new_uid != caller_uid || // you can't chown to someone else
> > inode_uid != caller_uid) { // you can't chown from someone else
> > continue;
> > }
> > }
> >
> > Shouldn't this just be a check for whether the caller_uid is 0 (or
> > whatever the correct check for the equivalent to the kernel's CAP_CHOWN
> > would be)?
Oops, I skipped over this part ^
> Without context, this does look a little weird — does it allow *any*
> change, given caller_uid needs to match both new and inode uid?
> Sort of the common case would be that the admin cap gets hit toward
> the beginning of the function and just allows it without ever reaching
> this point.
Yeah, the check is a bit weird. It looks like
1- A normal cap that specifies a uid can't ever change the uid. This
conditional could be simplified/clarified...
2- If you have a pair of caps, like
allow * uid=1, allow * uid=2
we still don't let you chown between uid 1 and 2. Well, not as caller_uid
1 or 2 (which is fine), but
3- Jeff is right, we don't allow root to chown between allowed uids.
Like if you had
allow * uid=0
shouldn't that let you chown anything? I didn't really consider this
case since most users would just do
allow *
which can do anything (including chown). But probably the 'allow * uid=0'
case should be handled properly.
sage
