Re: WARNING: bad usercopy in fanotify_read

[Kees Cook]

On Wed, Mar 13, 2019 at 7:35 AM Jan Kara <jack@xxxxxxx> wrote:
> On Tue 12-03-19 23:26:22, Kees Cook wrote:
> > On Mon, Mar 11, 2019 at 1:42 PM syzbot
> > <syzbot+2c49971e251e36216d1f@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17ee410b200000
> > > [...]
> > > ------------[ cut here ]------------
> > > Bad or missing usercopy whitelist? Kernel memory exposure attempt detected
> > > from SLAB object 'fanotify_event' (offset 40, size 8)!
> > > [...]
> > >   copy_to_user include/linux/uaccess.h:151 [inline]
> > >   copy_fid_to_user fs/notify/fanotify/fanotify_user.c:236 [inline]
> > >   copy_event_to_user fs/notify/fanotify/fanotify_user.c:294 [inline]
> >
> > Looks like this is the fh/ext_fh union in struct fanotify_fid, field
> > "fid" in struct fanotify_event. Given that "fid" is itself in a union
> > against a struct path, I think instead of a whitelist using
> > KMEM_CACHE_USERCOPY(), this should just use a bounce buffer to avoid
> > leaving a whitelist open for path or ext_fh exposure.
>
> Do you mean to protect it from a situation when some other code (i.e. not
> copy_fid_to_user()) would be tricked into copying ext_fh containing slab
> pointer to userspace?

Yes. That's the design around the usercopy hardening. The
"whitelisting" is either via code (with a bounce buffer, so only the
specific "expected" code path can copy it), with a
kmem_create_usercopy() range marking (generally best for areas that
are not unions or when bounce buffers would be too big/slow), or with
implicit whitelisting (via a constant copy size that cannot change at
run-time, like: copy_to_user(dst, src, 6)).

In this case, since there are multiple unions in place and
FANOTIFY_INLINE_FH_LEN is small, it seemed best to go with a bounce
buffer.

-- 
Kees Cook