On Mon, 2016-11-07 at 20:09 +0000, Sage Weil wrote:
> 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
It still seems to me like that should just be a check for superuser
status. Something like:
if (mask & MAY_CHOWN) {
// only root can chown
if (i->match.uid != 0 || caller_uid != 0)
continue;
}
}
i.e. only allow chown if the capability has a uid of 0 and the
caller_uid is also 0.
I don't think we want to ever grant an unprivileged user the ability to
chown, do we?
--
Jeff Layton <jlayton@xxxxxxxxxx>
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