Re: [PATCH] virtiofsd: Don't allow file creation with FUSE_OPEN

[Greg Kurz <groug@kaod.org>]

On Thu, 17 Jun 2021 15:29:12 +0100
"Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:

> * Greg Kurz (groug@kaod.org) wrote:
> > A well behaved FUSE client uses FUSE_CREATE to create files. It isn't
> > supposed to pass O_CREAT along a FUSE_OPEN request, as documented in
> > the "fuse_lowlevel.h" header :
> > 
> >     /**
> >      * Open a file
> >      *
> >      * Open flags are available in fi->flags. The following rules
> >      * apply.
> >      *
> >      *  - Creation (O_CREAT, O_EXCL, O_NOCTTY) flags will be
> >      *    filtered out / handled by the kernel.
> > 
> > But if it does anyway, virtiofsd crashes with:
> > 
> > *** invalid openat64 call: O_CREAT or O_TMPFILE without mode ***: terminated
> > 
> > This is because virtiofsd ends up passing this flag to openat() without
> > passing a mode_t 4th argument which is mandatory with O_CREAT, and glibc
> > aborts.
> > 
> > The offending path is:
> > 
> > lo_open()
> >     lo_do_open()
> >         lo_inode_open()
> > 
> > Other callers of lo_inode_open() only pass O_RDWR and lo_create()
> > passes a valid fd to lo_do_open() which thus doesn't even call
> > lo_inode_open() in this case.
> > 
> > Specifying O_CREAT with FUSE_OPEN is a protocol violation. Check this
> > in lo_open() and return an error to the client : EINVAL since this is
> > already what glibc returns with other illegal flag combinations.
> > 
> > The FUSE filesystem doesn't currently support O_TMPFILE, but the very
> > same would happen if O_TMPFILE was passed in a FUSE_OPEN request. Check
> > that as well.
> > 
> > Signed-off-by: Greg Kurz <groug@kaod.org>
> 
> That seems reasonable, so
> 
> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> 
> should you also send a corresponding patch for libfuse's example code
> we're based on?
> 

Good idea. I've just done that.

Cheers,

--
Greg

> Dave
> 
> > ---
> >  tools/virtiofsd/passthrough_ll.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
> > index 49c21fd85570..14f62133131c 100644
> > --- a/tools/virtiofsd/passthrough_ll.c
> > +++ b/tools/virtiofsd/passthrough_ll.c
> > @@ -2145,6 +2145,12 @@ static void lo_open(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi)
> >          return;
> >      }
> >  
> > +    /* File creation is handled by lo_create() */
> > +    if (fi->flags & (O_CREAT | O_TMPFILE)) {
> > +        fuse_reply_err(req, EINVAL);
> > +        return;
> > +    }
> > +
> >      err = lo_do_open(lo, inode, -1, fi);
> >      lo_inode_put(lo, &inode);
> >      if (err) {
> > -- 
> > 2.31.1
> >