Re: [PATCH] net: compat: fix msg_controllen overflow in scm_detach_fds_compat()

[Yang Xiao]

On Tue, Jun 4, 2019 at 8:46 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
>
> On 06/04/2019 02:31 PM, Young Xiao wrote:
> > There is a missing check between kmsg->msg_controllen and cmlen,
> > which can possibly lead to overflow.
> >
> > This bug is similar to vulnerability that was fixed in commit 6900317f5eff
> > ("net, scm: fix PaX detected msg_controllen overflow in scm_detach_fds").
>
> Back then I mentioned in commit 6900317f5eff:
>
>     In case of MSG_CMSG_COMPAT (scm_detach_fds_compat()), I haven't seen an
>     issue in my tests as alignment seems always on 4 byte boundary. Same
>     should be in case of native 32 bit, where we end up with 4 byte boundaries
>     as well.
>
> Do you have an actual reproducer or is it based on code inspection?

based on inspection.
>
> > Signed-off-by: Young Xiao <92siuyang@xxxxxxxxx>
> > ---
> >  net/compat.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/net/compat.c b/net/compat.c
> > index a031bd3..8e74dfb 100644
> > --- a/net/compat.c
> > +++ b/net/compat.c
> > @@ -301,6 +301,8 @@ void scm_detach_fds_compat(struct msghdr *kmsg, struct scm_cookie *scm)
> >                       err = put_user(cmlen, &cm->cmsg_len);
> >               if (!err) {
> >                       cmlen = CMSG_COMPAT_SPACE(i * sizeof(int));
> > +                     if (kmsg->msg_controllen < cmlen)
> > +                             cmlen = kmsg->msg_controllen;
> >                       kmsg->msg_control += cmlen;
> >                       kmsg->msg_controllen -= cmlen;
> >               }
> >
>


-- 
Best regards!

Young
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