[Virtio-fs] [PATCH v4 3/3] virtiofsd: prevent opening of special files (CVE-2020-35517)
Stefan Hajnoczi
stefanha at redhat.com
Wed Feb 3 17:05:14 UTC 2021
On Wed, Feb 03, 2021 at 11:08:58AM -0500, Vivek Goyal wrote:
> 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 at 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.
I thought about De Morgan's law too but found the OR expression is not
easier to read than the AND expression :(. If you prefer it written this
way I can change it though.
Stefan
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