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

[Richard Guy Briggs]

On 2020-04-20 17:56, Lenny Bruzenak wrote:
> On 4/20/20 5:29 PM, Paul Moore wrote:
> 
> > 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?
> 
> Not really. Using the libaudit interface(s), even if an empty message string
> is sent, and handled in the lib call(s), I believe it will have minimum
> contextual info, e.g. "exe=... hostname=... ", etc.
> 
> I can't think of a valid reason myself, assuming IIUC.

But even with an empty message, there is still pid, uid, auid, ses, subj
added by the kernel with its own message type.  I could see a valid type
of user message created that has no need for any of those fields added
by audit_log_user_message(), calling audit_send_user_message() directly
(but mind you, it appears to be deprecated to call it directly).

> LCB

- RGB

--
Richard Guy Briggs <rgb at redhat.com>
Sr. S/W Engineer, Kernel Security, Base Operating Systems
Remote, Ottawa, Red Hat Canada
IRC: rgb, SunRaycer
Voice: +1.647.777.2635, Internal: (81) 32635