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

[Greg Kurz]

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) {


>            goto out;
>        }
> > +        goto out;
> > +    }
> 
> 
> Vivek
>