Re: [PATCH 4.4 34/47] userns: move user access out of the mutex

[Greg KH]

On Sun, Sep 09, 2018 at 12:56:45AM -0300, Rafael David Tinoco wrote:
> Greg,
> 
> On Fri, Sep 7, 2018 at 6:41 PM Greg Kroah-Hartman
> <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> >
> > 4.4-stable review patch.  If anyone has any objections, please let me know.
> >
> > ------------------
> >
> > From: Jann Horn <jannh@xxxxxxxxxx>
> >
> > commit 5820f140edef111a9ea2ef414ab2428b8cb805b1 upstream.
> >
> > The old code would hold the userns_state_mutex indefinitely if
> > memdup_user_nul stalled due to e.g. a userfault region. Prevent that by
> > moving the memdup_user_nul in front of the mutex_lock().
> >
> > Note: This changes the error precedence of invalid buf/count/*ppos vs
> > map already written / capabilities missing.
> >
> > Fixes: 22d917d80e84 ("userns: Rework the user_namespace adding uid/gid...")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> > Acked-by: Christian Brauner <christian@xxxxxxxxxx>
> > Acked-by: Serge Hallyn <serge@xxxxxxxxxx>
> > Signed-off-by: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> >
> > ---
> >  kernel/user_namespace.c |   22 ++++++++++------------
> >  1 file changed, 10 insertions(+), 12 deletions(-)
> >
> > --- a/kernel/user_namespace.c
> > +++ b/kernel/user_namespace.c
> > @@ -604,7 +604,16 @@ static ssize_t map_write(struct file *fi
> >         struct uid_gid_extent *extent = NULL;
> >         unsigned long page = 0;
> >         char *kbuf, *pos, *next_line;
> > -       ssize_t ret = -EINVAL;
> > +       ssize_t ret;
> > +
> > +       /* Only allow < page size writes at the beginning of the file */
> > +       if ((*ppos != 0) || (count >= PAGE_SIZE))
> > +               return -EINVAL;
> > +
> > +       /* Slurp in the user data */
> > +       if (copy_from_user(kbuf, buf, count))
> > +               return -EFAULT;
> > +       kbuf[count] = '\0';
> 
> Naresh will soon report issues found by LKFT on user_ns for 4.4 kernel
> for this review round.
> 
> selftests: mount_run_tests.sh [FAIL]
> write to /proc/self/uid_map failed: Bad address
> 
> LTP: user_namespace2 1 TBROK : safe_macros.c:452: userns02.c:95:
> write(6,0x7ffc133113d0,18446744073709551615) failed: errno=EFAULT(14):
> Bad address
> 
> I believe the EFAULT was caused because when changing code from
> "memdup_user_nul" to "copy_from_user", for the older kernels, you
> missed allocating the slab object for "kbuf", like memdup_user_nul()
> does.
> 
> Note: This likely applies to 3.18 as well.
> 
> We are finishing functional tests without this patch, but we wanted to
> make you aware right away.

Nice catch.  Ugh, that's all my fault for when I backported this.  Let
me go work on that now...

greg k-h