Re: [PATCH v4 3/3] virtiofsd: prevent opening of special files (CVE-2020-35517)

[Vivek Goyal]

On Wed, Feb 03, 2021 at 05:02:37PM +0100, Greg Kurz wrote:
> On Wed, 3 Feb 2021 10:28:50 -0500
> Vivek Goyal <vgoyal@redhat.com> wrote:
> 
> > On Wed, Feb 03, 2021 at 11:37:19AM +0000, Stefan Hajnoczi wrote:
> > 
> > [..]
> > > @@ -1727,36 +1764,38 @@ static void lo_create(fuse_req_t req, fuse_ino_t 
> > > parent, const char *name,
> > >  
> > >      update_open_flags(lo->writeback, lo->allow_direct_io, fi);
> > >  
> > > -    fd = openat(parent_inode->fd, name, (fi->flags | O_CREAT) & 
> > > ~O_NOFOLLOW,
> > > -                mode);
> > > +    /* Try to create a new file but don't open existing files */
> > > +    fd = openat(parent_inode->fd, name, fi->flags | O_CREAT | O_EXCL, 
> > > mode);
> > >      err = fd == -1 ? errno : 0;
> > > +
> > >      lo_restore_cred(&old);
> > >  
> > > -    if (!err) {
> > > -        ssize_t fh;
> > > -
> > > -        pthread_mutex_lock(&lo->mutex);
> > > -        fh = lo_add_fd_mapping(lo, fd);
> > > -        pthread_mutex_unlock(&lo->mutex);
> > > -        if (fh == -1) {
> > > -            close(fd);
> > > -            err = ENOMEM;
> > > -            goto out;
> > > -        }
> > > +    /* Ignore the error if file exists and O_EXCL was not given */
> > > +    if (err && !(err == EEXIST && !(fi->flags & O_EXCL))) {
> > 
> > Can this check be simplified to.
> >        if (err && (err == EEXIST && (fi->flags & O_EXCL)) {
> 
> I guess you meant :
> 
>         if (err && (err != EEXIST || fi->flags & O_EXCL) {

This sounds correct. I forgot to take into account that if error is
not -EEXIST, we still want to bail out irrespective of O_EXCL.

Vivek