From: Kees Cook <keescook@chromium.org>
To: Jan Kara <jack@suse.cz>
Cc: syzbot <syzbot+2c49971e251e36216d1f@syzkaller.appspotmail.com>,
Amir Goldstein <amir73il@gmail.com>,
Andrew Morton <akpm@linux-foundation.org>,
cai@lca.pw, Chris von Recklinghausen <crecklin@redhat.com>,
LKML <linux-kernel@vger.kernel.org>,
Linux-MM <linux-mm@kvack.org>,
syzkaller-bugs <syzkaller-bugs@googlegroups.com>
Subject: Re: WARNING: bad usercopy in fanotify_read
Date: Wed, 13 Mar 2019 08:35:33 -0700 [thread overview]
Message-ID: <CAGXu5j+_Ao_CU8DG9nrTbx5ioDkJUFw0cGcLBMWnvNLe_eFJ4A@mail.gmail.com> (raw)
In-Reply-To: <20190313143503.GD9108@quack2.suse.cz>
On Wed, Mar 13, 2019 at 7:35 AM Jan Kara <jack@suse.cz> wrote:
> On Tue 12-03-19 23:26:22, Kees Cook wrote:
> > On Mon, Mar 11, 2019 at 1:42 PM syzbot
> > <syzbot+2c49971e251e36216d1f@syzkaller.appspotmail.com> 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
next prev parent reply other threads:[~2019-03-13 15:35 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-11 20:42 WARNING: bad usercopy in fanotify_read syzbot
2019-03-13 6:26 ` Kees Cook
2019-03-13 6:42 ` Amir Goldstein
2019-03-13 14:35 ` Jan Kara
2019-03-13 15:35 ` Kees Cook [this message]
2019-03-13 15:47 ` Jan Kara
2019-03-18 18:27 ` Kees Cook
2019-03-19 8:32 ` Jan Kara
2019-03-16 14:49 ` syzbot
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