Re: [PATCH] virtiofs: Enable SB_NOSEC flag to improve small write performance

[Vivek Goyal <vgoyal@redhat.com>]

On Tue, Jul 21, 2020 at 11:55:03AM -0400, Vivek Goyal wrote:
> On Tue, Jul 21, 2020 at 05:44:14PM +0200, Miklos Szeredi wrote:
> > On Tue, Jul 21, 2020 at 5:17 PM Vivek Goyal <vgoyal@redhat.com> wrote:
> > >
> > > On Tue, Jul 21, 2020 at 02:33:41PM +0200, Miklos Szeredi wrote:
> > > > On Mon, Jul 20, 2020 at 5:41 PM Vivek Goyal <vgoyal@redhat.com> wrote:
> > > > >
> > > > > On Fri, Jul 17, 2020 at 10:53:07AM +0200, Miklos Szeredi wrote:
> > > >
> > > > > I see in VFS that chown() always kills suid/sgid. While truncate() and
> > > > > write(), will suid/sgid only if caller does not have CAP_FSETID.
> > > > >
> > > > > How does this work with FUSE_HANDLE_KILLPRIV. IIUC, file server does not
> > > > > know if caller has CAP_FSETID or not. That means file server will be
> > > > > forced to kill suid/sgid on every write and truncate. And that will fail
> > > > > some of the tests.
> > > > >
> > > > > For WRITE requests now we do have the notion of setting
> > > > > FUSE_WRITE_KILL_PRIV flag to tell server explicitly to kill suid/sgid.
> > > > > Probably we could use that in cached write path as well to figure out
> > > > > whether to kill suid/sgid or not. But truncate() will still continue
> > > > > to be an issue.
> > > >
> > > > Yes, not doing the same for truncate seems to be an oversight.
> > > > Unfortunate, since we'll need another INIT flag to enable selective
> > > > clearing of suid/sgid on truncate.
> > > >
> > > > >
> > > > > >
> > > > > > Even writeback_cache could be handled by this addition, since we call
> > > > > > fuse_update_attributes() before generic_file_write_iter() :
> > > > > >
> > > > > > --- a/fs/fuse/dir.c
> > > > > > +++ b/fs/fuse/dir.c
> > > > > > @@ -985,6 +985,7 @@ static int fuse_update_get_attr(struct inode
> > > > > > *inode, struct file *file,
> > > > > >
> > > > > >         if (sync) {
> > > > > >                 forget_all_cached_acls(inode);
> > > > > > +               inode->i_flags &= ~S_NOSEC;
> > > > >
> > > > > Ok, So I was clearing S_NOSEC only if server reports that file has
> > > > > suid/sgid bit set. This change will clear S_NOSEC whenever we fetch
> > > > > attrs from host and will force getxattr() when we call file_remove_privs()
> > > > > and will increase overhead for non cache writeback mode. We probably
> > > > > could keep both. For cache writeback mode, clear it undonditionally
> > > > > otherwise not.
> > > >
> > > > We clear S_NOSEC because the attribute timeout has expired.  This
> > > > means we need to refresh all metadata, including cached xattr (which
> > > > is what S_NOSEC effectively is).
> > > >
> > > > > What I don't understand is though that how this change will clear
> > > > > suid/sgid on host in cache=writeback mode. I see fuse_setattr()
> > > > > will not set ATTR_MODE and clear S_ISUID and S_ISGID if
> > > > > fc->handle_killpriv is set. So when server receives setattr request
> > > > > (if it does), then how will it know it is supposed to kill suid/sgid
> > > > > bit. (its not chown, truncate and its not write).
> > > >
> > > > Depends.  If the attribute timeout is infinity, then that means the
> > > > cache is always up to date.  In that case we only need to clear
> > > > suid/sgid if set in i_mode.  Similarly, the security.capability will
> > > > only be cleared if it was set in the first place (which would clear
> > > > S_NOSEC).
> > > >
> > > > If the timeout is finite, then that means we need to check if the
> > > > metadata changed after a timeout.  That's the purpose of the
> > > > fuse_update_attributes() call before generic_file_write_iter().
> > > >
> > > > Does that make it clear?
> > >
> > > I understood it partly but one thing is still bothering me. What
> > > happens when cache writeback is set as well as fc->handle_killpriv=1.
> > >
> > > When handle_killpriv is set, how suid/sgid will be cleared by
> > > server. Given cache=writeback, write probably got cached in
> > > guest and server probably will not not see a WRITE immideately.
> > > (I am assuming we are relying on a WRITE to clear setuid/setgid when
> > >  handle_killpriv is set). And that means server will not clear
> > >  setuid/setgid till inode is written back at some point of time
> > >  later.
> > >
> > > IOW, cache=writeback and fc->handle_killpriv don't seem to go
> > > together (atleast given the current code).
> > 
> > fuse_cache_write_iter()
> >   -> fuse_update_attributes()   * this will refresh i_mode
> >   -> generic_file_write_iter()
> >       ->__generic_file_write_iter()
> >           ->file_remove_privs()    * this will check i_mode
> >               ->__remove_privs()
> >                   -> notify_change()
> >                      -> fuse_setattr()   * this will clear suid/sgit bits
> 
> And fuse_setattr() has following.
> 
>                 if (!fc->handle_killpriv) {
>                         /*
>                          * ia_mode calculation may have used stale i_mode.
>                          * Refresh and recalculate.
>                          */
>                         ret = fuse_do_getattr(inode, NULL, file);
>                         if (ret)
>                                 return ret;
> 
>                         attr->ia_mode = inode->i_mode;
>                         if (inode->i_mode & S_ISUID) {
>                                 attr->ia_valid |= ATTR_MODE;
>                                 attr->ia_mode &= ~S_ISUID;
>                         }
>                         if ((inode->i_mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
>                                 attr->ia_valid |= ATTR_MODE;
>                                 attr->ia_mode &= ~S_ISGID;
>                         }
>                 }
>         }
>         if (!attr->ia_valid)
>                 return 0;
> 
> So if fc->handle_killpriv is set, we might not even send setattr
> request if attr->ia_valid turns out to be zero.
> 
> I did a quick instrumentation and noticed that we are sending
> setattr with attr->ia_valid=0x200 (ATTR_FORCE) set. And file
> server is not required to kill suid/sgid in this case?

Did little more instrumentation of fuse and virtiofsd. Modified 
virtiofsd to enable FUSE_HANDLE_KILLPRIV and ran virtiofsd with
-o writeback.

On client created a file /mnt/virtiofs/foo.txt and set setuid bit.
Write a program to write a single charater to the file and
dropped CAP_FSETID before executing the program and noticed messages
coming on virtiofsd. 

I see no WRITE came and lo_setattr() was called with valid=0x0. And
that means it will not change any of the attrs and simply get
current attrs and return to client.

A WRITE comes later either when file is close (fuse_flush()) or
a writeback is triggred. So if file server clears setuid/setgid
bit always on WRITE, then setuid/setgid bit will ultimately
be cleared but much later when guest page is written back.

Hopefully I am not missing something very basic.

Thanks
Vivek