[PATCH] audit: check the length of userspace generated audit records

[Paul Moore]

On Mon, Apr 20, 2020 at 5:56 PM Richard Guy Briggs <rgb at redhat.com> wrote:
> On 2020-04-20 17:36, Paul Moore wrote:
> > Commit 756125289285 ("audit: always check the netlink payload length
> > in audit_receive_msg()") fixed a number of missing message length
> > checks, but forgot to check the length of userspace generated audit
> > records.  The good news is that you need CAP_AUDIT_WRITE to submit
> > userspace audit records, which is generally only given to trusted
> > processes, so the impact should be limited.
> >
> > Cc: stable at vger.kernel.org
> > Fixes: 756125289285 ("audit: always check the netlink payload length in audit_receive_msg()")
> > Reported-by: syzbot+49e69b4d71a420ceda3e at syzkaller.appspotmail.com
> > Signed-off-by: Paul Moore <paul at paul-moore.com>
> > ---
> >  kernel/audit.c |    3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/kernel/audit.c b/kernel/audit.c
> > index b69c8b460341..87f31bf1f0a0 100644
> > --- a/kernel/audit.c
> > +++ b/kernel/audit.c
> > @@ -1326,6 +1326,9 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
> >       case AUDIT_FIRST_USER_MSG2 ... AUDIT_LAST_USER_MSG2:
> >               if (!audit_enabled && msg_type != AUDIT_USER_AVC)
> >                       return 0;
> > +             /* exit early if there isn't at least one character to print */
> > +             if (data_len < 2)
> > +                     return -EINVAL;
>
> Don't we want to issue the record even if the message is empty?  If a
> len of 1 is passed in, it will properly set str[0] = '\0' and str points
> to a location with a null that prints nothing between the single quotes
> of "msg=''".  So, I think that should be "if (data_len < 1)".
>
> Am I missing something?

I've got no problem with allowing an empty message so long as there is
a valid use for such a message.  Can anyone think of a valid reason
for having an empty userspace generated record?

-- 
paul moore
www.paul-moore.com